diff --git a/packages/v4/migration/Action/Action.spec.ts b/packages/v4/migration/Action/Action.spec.ts
index ab7e5e2b5..12d9a633c 100644
--- a/packages/v4/migration/Action/Action.spec.ts
+++ b/packages/v4/migration/Action/Action.spec.ts
@@ -7,7 +7,7 @@ import {
   SWAP_MODES,
   type BaseConfig,
 } from '../../src/index.js';
-import { mount, resetDom, settle } from '../../src/test/index.js';
+import { captureDiagnostics, mount, resetDom, settle } from '../../src/test/index.js';
 import { Dialog } from '../Dialog/Dialog.js';
 import { Action } from './Action.js';
 import { ActionEvent } from './ActionEvent.js';
@@ -43,7 +43,7 @@ registerComponents(Action, Target, Foo, Bar, Dialog, MountProbe);
 afterEach(resetDom);
 
 function at<T extends Base>(root: ParentNode, selector: string, name: string): T {
-  return getInstance<T>(root.querySelector(selector)!, name)!;
+  return getInstance<T>(root.querySelector(selector), name)!;
 }
 
 function click(el: Element): Event {
@@ -464,27 +464,22 @@ describe('Action — the component', () => {
     const root = await mount(`
       <div id="action" data-component="Action" data-on:click="() => consol.log()"></div>
     `);
-    const details: Array<Record<string, unknown>> = [];
-    // Canceling suppresses the default sink, so the failure does not reach
-    // `reportError()` and fail the run.
-    const listener = (event: Event) => {
-      details.push((event as CustomEvent<Record<string, unknown>>).detail);
-      event.preventDefault();
-    };
-    document.addEventListener('js-toolkit:diagnostic', listener);
+    // The capture cancels every event, which suppresses the default sink, so
+    // the failure never reaches `reportError()` and fails the run.
+    const log = captureDiagnostics();
 
     click(root.querySelector('#action') as Element);
 
-    expect(details).toHaveLength(1);
-    expect(details[0]).toMatchObject({
+    expect(log.entries).toHaveLength(1);
+    expect(log.entries[0]).toMatchObject({
       severity: 'error',
       code: 'action.effect-failed',
       component: 'Action',
     });
     // The cause survives, which a bare `console.warn` never carried.
-    expect(details[0].error).toBeInstanceOf(Error);
+    expect(log.entries[0].error).toBeInstanceOf(Error);
 
-    document.removeEventListener('js-toolkit:diagnostic', listener);
+    log.stop();
   });
 });
 
diff --git a/packages/v4/migration/AnchorNav/AnchorNav.spec.ts b/packages/v4/migration/AnchorNav/AnchorNav.spec.ts
index dbf14befc..61e40606b 100644
--- a/packages/v4/migration/AnchorNav/AnchorNav.spec.ts
+++ b/packages/v4/migration/AnchorNav/AnchorNav.spec.ts
@@ -25,7 +25,7 @@ describe('AnchorNav', () => {
   it('enters the matching link once its target scrolls into view', async () => {
     const { root, target } = await render();
     const link = getInstance<AnchorNavLink>(
-      root.querySelector('[data-component="AnchorNavLink"]')!,
+      root.querySelector('[data-component="AnchorNavLink"]'),
       'AnchorNavLink',
     )!;
 
@@ -41,7 +41,7 @@ describe('AnchorNav', () => {
   it('leaves the matching link once its target scrolls back out of view', async () => {
     const { root, target } = await render();
     const link = getInstance<AnchorNavLink>(
-      root.querySelector('[data-component="AnchorNavLink"]')!,
+      root.querySelector('[data-component="AnchorNavLink"]'),
       'AnchorNavLink',
     )!;
 
@@ -66,7 +66,7 @@ describe('AnchorNav', () => {
     document.body.append(root);
     await settle();
     const link = getInstance<AnchorNavLink>(
-      root.querySelector('[data-component="AnchorNavLink"]')!,
+      root.querySelector('[data-component="AnchorNavLink"]'),
       'AnchorNavLink',
     )!;
     const target = root.querySelector('#one') as HTMLElement;
diff --git a/packages/v4/migration/AnchorNav/AnchorNavLink.spec.ts b/packages/v4/migration/AnchorNav/AnchorNavLink.spec.ts
index fbfbee046..08a0f8cb1 100644
--- a/packages/v4/migration/AnchorNav/AnchorNavLink.spec.ts
+++ b/packages/v4/migration/AnchorNav/AnchorNavLink.spec.ts
@@ -23,7 +23,7 @@ async function render(): Promise<AnchorNavLink> {
   root.innerHTML = `<a data-component="AnchorNavLink" href="#section-one" ${OPTIONS_ATTRS}></a>`;
   document.body.append(root);
   await settle();
-  return getInstance<AnchorNavLink>(root.firstElementChild!, 'AnchorNavLink')!;
+  return getInstance<AnchorNavLink>(root.firstElementChild, 'AnchorNavLink')!;
 }
 
 describe('AnchorNavLink', () => {
diff --git a/packages/v4/migration/AnchorNav/AnchorNavTarget.spec.ts b/packages/v4/migration/AnchorNav/AnchorNavTarget.spec.ts
index f52a39b32..886544ac4 100644
--- a/packages/v4/migration/AnchorNav/AnchorNavTarget.spec.ts
+++ b/packages/v4/migration/AnchorNav/AnchorNavTarget.spec.ts
@@ -1,6 +1,5 @@
 import { afterEach, describe, expect, it } from 'vitest';
-import { registerComponents } from '../../src/index.js';
-import { INSTANCES } from '../../src/protocol-symbols.js';
+import { getInstance, registerComponents } from '../../src/index.js';
 import { resetDom, settle, waitFor } from '../../src/test/index.js';
 import { AnchorNavTarget } from './AnchorNavTarget.js';
 
@@ -18,7 +17,7 @@ async function quiet(): Promise<void> {
   }
 }
 
-const mountedState = (el: HTMLElement) => el[INSTANCES]?.get('AnchorNavTarget')?.$isMounted;
+const mountedState = (el: HTMLElement) => getInstance(el, 'AnchorNavTarget')?.$isMounted;
 
 function render(style: string): HTMLElement {
   const el = document.createElement('div');
diff --git a/packages/v4/migration/ClickOutside/ClickOutside.spec.ts b/packages/v4/migration/ClickOutside/ClickOutside.spec.ts
index 717867f65..8bb95eefc 100644
--- a/packages/v4/migration/ClickOutside/ClickOutside.spec.ts
+++ b/packages/v4/migration/ClickOutside/ClickOutside.spec.ts
@@ -79,7 +79,7 @@ describe('ClickOutside', () => {
     outside.click();
 
     const dropdown = getInstance<Dropdown>(
-      root.querySelector('[data-component="Dropdown"]')!,
+      root.querySelector('[data-component="Dropdown"]'),
       'Dropdown',
     )!;
     expect(dropdown.closed).toHaveLength(1);
diff --git a/packages/v4/migration/Data/DataBind.spec.ts b/packages/v4/migration/Data/DataBind.spec.ts
index 7b71708b1..661dbc890 100644
--- a/packages/v4/migration/Data/DataBind.spec.ts
+++ b/packages/v4/migration/Data/DataBind.spec.ts
@@ -67,7 +67,7 @@ function el<T extends HTMLElement = HTMLElement>(root: HTMLElement, selector: st
 }
 
 function at<T>(root: HTMLElement, selector: string, name: string): T {
-  return getInstance<never>(root.querySelector(selector)!, name)! as T;
+  return getInstance<never>(root.querySelector(selector), name)! as T;
 }
 
 describe('DataBind — the element half', () => {
@@ -272,15 +272,9 @@ describe('DataBind — the element half', () => {
   });
 
   it('warns for a binding type that names nothing', async () => {
-    const details: string[] = [];
-    // Removed at the end: this listener cancels the default sink, and leaking
-    // it silenced every later diagnostic in the file.
-    const listener = (event: Event) => {
-      const { detail } = event as CustomEvent<{ code: string; message: string }>;
-      details.push(detail.code);
-      event.preventDefault();
-    };
-    document.addEventListener(EVENTS.diagnostic, listener);
+    // Stopped at the end: the capture cancels the default sink, and leaking it
+    // silenced every later diagnostic in the file.
+    const log = captureDiagnostics();
 
     const root = await mount(`
       <div id="d" data-component="DataBind" data-option-group="${uniqueGroup('v')}"
@@ -288,10 +282,10 @@ describe('DataBind — the element half', () => {
     `);
 
     // The typo used to be an attribute that silently did nothing at all.
-    expect(details).toContain('attribute.unknown-qualifier');
+    expect(log.codes).toContain('attribute.unknown-qualifier');
     expect(at<DataBind>(root, '#d', 'DataBind').hasVirtualBindings).toBe(false);
 
-    document.removeEventListener(EVENTS.diagnostic, listener);
+    log.stop();
   });
 
   it('fails quietly when a virtual expression throws', async () => {
diff --git a/packages/v4/migration/Data/DataDerived.spec.ts b/packages/v4/migration/Data/DataDerived.spec.ts
index 3f95defc3..8940d2ed9 100644
--- a/packages/v4/migration/Data/DataDerived.spec.ts
+++ b/packages/v4/migration/Data/DataDerived.spec.ts
@@ -30,7 +30,7 @@ function el<T extends HTMLElement = HTMLElement>(root: HTMLElement, selector: st
 }
 
 function at<T>(root: HTMLElement, selector: string, name: string): T {
-  return getInstance<never>(root.querySelector(selector)!, name)! as T;
+  return getInstance<never>(root.querySelector(selector), name)! as T;
 }
 
 describe('DataModel', () => {
diff --git a/packages/v4/migration/Data/DataScope.spec.ts b/packages/v4/migration/Data/DataScope.spec.ts
index a4e3ee1c2..304d0ea53 100644
--- a/packages/v4/migration/Data/DataScope.spec.ts
+++ b/packages/v4/migration/Data/DataScope.spec.ts
@@ -27,7 +27,7 @@ function uniqueGroup(name: string): string {
 }
 
 function at<T>(root: HTMLElement, selector: string, name: string): T {
-  return getInstance<never>(root.querySelector(selector)!, name)! as T;
+  return getInstance<never>(root.querySelector(selector), name)! as T;
 }
 
 function el<T extends HTMLElement = HTMLElement>(root: HTMLElement, selector: string): T {
diff --git a/packages/v4/migration/Fetch/FetchShopifyPartial.spec.ts b/packages/v4/migration/Fetch/FetchShopifyPartial.spec.ts
index 7717fc765..78e5b6ed0 100644
--- a/packages/v4/migration/Fetch/FetchShopifyPartial.spec.ts
+++ b/packages/v4/migration/Fetch/FetchShopifyPartial.spec.ts
@@ -1,6 +1,6 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import { getInstance, registerComponents } from '../../src/index.js';
-import { recordEvents, resetDom, settle } from '../../src/test/index.js';
+import { mount, recordEvents, resetDom, settle } from '../../src/test/index.js';
 import { FETCH_EVENTS } from './Fetch.js';
 import { FetchShopifyPartial } from './FetchShopifyPartial.js';
 
@@ -27,13 +27,15 @@ afterEach(async () => {
   await resetDom();
 });
 
-async function mount(html: string): Promise<{ root: HTMLElement; instance: FetchShopifyPartial }> {
-  const root = document.createElement('div');
-  root.innerHTML = html;
-  document.body.append(root);
-  await settle();
-  const el = root.firstElementChild as HTMLElement;
-  return { root, instance: getInstance<FetchShopifyPartial>(el, 'FetchShopifyPartial')! };
+/** {@link mount}, plus the one instance every test here goes on to drive. */
+async function mountWithInstance(
+  html: string,
+): Promise<{ root: HTMLElement; instance: FetchShopifyPartial }> {
+  const root = await mount(html);
+  return {
+    root,
+    instance: getInstance<FetchShopifyPartial>(root.firstElementChild, 'FetchShopifyPartial')!,
+  };
 }
 
 function stubClient(
@@ -54,7 +56,7 @@ function stubPartials(api: {
 describe('FetchShopifyPartial', () => {
   it('falls back to the base Fetch behaviour when no partials are configured', async () => {
     const client = stubClient();
-    const { root, instance } = await mount(
+    const { root, instance } = await mountWithInstance(
       `<a data-component="FetchShopifyPartial" href="/page" id="a"><div id="a">old</div></a>`,
     );
     const { events } = recordEvents(root, ...Object.values(FETCH_EVENTS));
@@ -71,7 +73,7 @@ describe('FetchShopifyPartial', () => {
     const apply = vi.fn();
     const fetchPartials = vi.fn(async () => ({ shape: 'partial-update' }));
     stubPartials({ fetch: fetchPartials, apply });
-    const { root, instance } = await mount(
+    const { root, instance } = await mountWithInstance(
       `<a data-component="FetchShopifyPartial" href="/page" data-option-partials="main, header"></a>`,
     );
     const { events } = recordEvents(root, ...Object.values(FETCH_EVENTS));
@@ -106,7 +108,7 @@ describe('FetchShopifyPartial', () => {
     FetchShopifyPartial.loadPartialsModule = async () => {
       throw new Error('not installed');
     };
-    const { instance } = await mount(
+    const { instance } = await mountWithInstance(
       `<a data-component="FetchShopifyPartial" href="/page" data-option-partials="main"><div id="a">old</div></a>`,
     );
 
@@ -120,7 +122,7 @@ describe('FetchShopifyPartial', () => {
     const client = stubClient();
     const fetchPartials = vi.fn(async () => ({}));
     stubPartials({ fetch: fetchPartials, apply: vi.fn() });
-    const { instance } = await mount(
+    const { instance } = await mountWithInstance(
       `<a data-component="FetchShopifyPartial" href="/page" data-option-partials="main"><div id="a">old</div></a>`,
     );
 
@@ -135,7 +137,7 @@ describe('FetchShopifyPartial', () => {
     const client = stubClient();
     const fetchPartials = vi.fn(async () => ({}));
     stubPartials({ fetch: fetchPartials, apply: vi.fn() });
-    const { instance } = await mount(
+    const { instance } = await mountWithInstance(
       `<a data-component="FetchShopifyPartial" href="/page" data-option-partials="main"><div id="a">old</div></a>`,
     );
 
@@ -150,7 +152,7 @@ describe('FetchShopifyPartial', () => {
     const client = stubClient();
     const fetchPartials = vi.fn(async () => ({}));
     stubPartials({ fetch: fetchPartials, apply: vi.fn() });
-    const { instance } = await mount(
+    const { instance } = await mountWithInstance(
       `<a data-component="FetchShopifyPartial" href="/page" data-option-partials="main"><div id="a">old</div></a>`,
     );
 
@@ -166,7 +168,7 @@ describe('FetchShopifyPartial', () => {
     const client = stubClient();
     const fetchPartials = vi.fn(async () => ({}));
     stubPartials({ fetch: fetchPartials, apply: vi.fn() });
-    const { instance } = await mount(
+    const { instance } = await mountWithInstance(
       `<a data-component="FetchShopifyPartial" href="/page" data-option-partials="main"><div id="a">old</div></a>`,
     );
 
@@ -181,7 +183,7 @@ describe('FetchShopifyPartial', () => {
     const client = stubClient();
     const fetchPartials = vi.fn(async () => ({}));
     stubPartials({ fetch: fetchPartials, apply: vi.fn() });
-    const { instance } = await mount(
+    const { instance } = await mountWithInstance(
       `<a data-component="FetchShopifyPartial" href="/page" data-option-partials="main"></a>`,
     );
 
@@ -199,7 +201,7 @@ describe('FetchShopifyPartial', () => {
       fetch: async () => ({}),
       apply: () => Promise.reject(failure),
     });
-    const { root, instance } = await mount(
+    const { root, instance } = await mountWithInstance(
       `<a data-component="FetchShopifyPartial" href="/page" data-option-partials="main"></a>`,
     );
     const errors: unknown[] = [];
@@ -215,7 +217,7 @@ describe('FetchShopifyPartial', () => {
 
   it('skips the history push for a popstate header given as a Headers instance', async () => {
     stubPartials({ fetch: async () => ({}), apply: vi.fn() });
-    const { instance } = await mount(
+    const { instance } = await mountWithInstance(
       `<a data-component="FetchShopifyPartial" href="/page" data-option-partials="main" data-option-history></a>`,
     );
     const before = window.history.length;
@@ -232,7 +234,7 @@ describe('FetchShopifyPartial', () => {
 
   it('still pushes history for a request that is not popstate-triggered', async () => {
     stubPartials({ fetch: async () => ({}), apply: vi.fn() });
-    const { instance } = await mount(
+    const { instance } = await mountWithInstance(
       `<a data-component="FetchShopifyPartial" href="/page" data-option-partials="main" data-option-history></a>`,
     );
     const before = window.history.length;
@@ -248,7 +250,7 @@ describe('FetchShopifyPartial', () => {
       partials: { fetch: vi.fn(async () => ({})), apply: vi.fn() },
     }));
     FetchShopifyPartial.loadPartialsModule = loadSpy;
-    const { instance } = await mount(
+    const { instance } = await mountWithInstance(
       `<a data-component="FetchShopifyPartial" href="/page" data-option-partials="main"></a>`,
     );
 
diff --git a/packages/v4/migration/Figure/FigureShopify.spec.ts b/packages/v4/migration/Figure/FigureShopify.spec.ts
index 0d9766c0b..9233ce424 100644
--- a/packages/v4/migration/Figure/FigureShopify.spec.ts
+++ b/packages/v4/migration/Figure/FigureShopify.spec.ts
@@ -19,7 +19,7 @@ async function render(attributes = ''): Promise<FigureShopify> {
     </div>`;
   document.body.append(root);
   await settle();
-  return getInstance<FigureShopify>(root.firstElementChild!, 'FigureShopify')!;
+  return getInstance<FigureShopify>(root.firstElementChild, 'FigureShopify')!;
 }
 
 describe('FigureShopify', () => {
diff --git a/packages/v4/migration/Figure/FigureTwicpics.spec.ts b/packages/v4/migration/Figure/FigureTwicpics.spec.ts
index c365a33b4..ccdc9ecaf 100644
--- a/packages/v4/migration/Figure/FigureTwicpics.spec.ts
+++ b/packages/v4/migration/Figure/FigureTwicpics.spec.ts
@@ -20,7 +20,7 @@ async function render(
     </div>`;
   document.body.append(root);
   await settle();
-  return getInstance<FigureTwicpics>(root.firstElementChild!, 'FigureTwicpics')!;
+  return getInstance<FigureTwicpics>(root.firstElementChild, 'FigureTwicpics')!;
 }
 
 describe('FigureTwicpics', () => {
diff --git a/packages/v4/migration/FigureVideo/FigureVideo.spec.ts b/packages/v4/migration/FigureVideo/FigureVideo.spec.ts
index 55d5e6735..fe7f29b68 100644
--- a/packages/v4/migration/FigureVideo/FigureVideo.spec.ts
+++ b/packages/v4/migration/FigureVideo/FigureVideo.spec.ts
@@ -100,24 +100,18 @@ describe('FigureVideo', () => {
 
   it('settles and reports when the sources fail, instead of hanging forever', async () => {
     const { el, video } = render(ONSCREEN);
-    const details: Array<Record<string, unknown>> = [];
-    const listener = (event: Event) => {
-      details.push((event as CustomEvent<Record<string, unknown>>).detail);
-      event.preventDefault();
-    };
-    document.addEventListener('js-toolkit:diagnostic', listener);
+    const log = captureDiagnostics();
 
     await settle();
     // v3 waits on `loadeddata` alone, so this never settled and `mounted()`
     // never returned.
     video.dispatchEvent(new Event('error'));
-    await waitFor(() => details.length > 0);
+    await waitFor(() => log.codes.includes('figure-video.load-failed'));
 
-    expect(details.map((detail) => detail.code)).toContain('figure-video.load-failed');
     // Left un-loaded, so a later mount cycle can retry.
     expect(getInstance<FigureVideo>(el, 'FigureVideo')!.hasLoaded).toBe(false);
 
-    document.removeEventListener('js-toolkit:diagnostic', listener);
+    log.stop();
   });
 
   it('warns and does not throw when the video ref is missing', async () => {
diff --git a/packages/v4/migration/FigureVideo/FigureVideoTwicpics.spec.ts b/packages/v4/migration/FigureVideo/FigureVideoTwicpics.spec.ts
index 060300f40..b035765c2 100644
--- a/packages/v4/migration/FigureVideo/FigureVideoTwicpics.spec.ts
+++ b/packages/v4/migration/FigureVideo/FigureVideoTwicpics.spec.ts
@@ -1,6 +1,6 @@
 import { afterEach, describe, expect, it } from 'vitest';
 import { getInstance, registerComponents } from '../../src/index.js';
-import { resetDom, settle } from '../../src/test/index.js';
+import { captureDiagnostics, resetDom, settle, waitFor } from '../../src/test/index.js';
 import { FigureVideoTwicpics } from './FigureVideoTwicpics.js';
 
 registerComponents(FigureVideoTwicpics);
@@ -20,7 +20,7 @@ async function render(attributes = ''): Promise<FigureVideoTwicpics> {
     </div>`;
   document.body.append(root);
   await settle();
-  return getInstance<FigureVideoTwicpics>(root.firstElementChild!, 'FigureVideoTwicpics')!;
+  return getInstance<FigureVideoTwicpics>(root.firstElementChild, 'FigureVideoTwicpics')!;
 }
 
 describe('FigureVideoTwicpics — the loadSources override', () => {
@@ -33,12 +33,7 @@ describe('FigureVideoTwicpics — the loadSources override', () => {
         </video>
       </div>`;
     document.body.append(root);
-    const details: Array<Record<string, unknown>> = [];
-    const listener = (event: Event) => {
-      details.push((event as CustomEvent<Record<string, unknown>>).detail);
-      event.preventDefault();
-    };
-    document.addEventListener('js-toolkit:diagnostic', listener);
+    const log = captureDiagnostics();
     await settle();
 
     const el = root.firstElementChild as HTMLElement;
@@ -46,14 +41,11 @@ describe('FigureVideoTwicpics — the loadSources override', () => {
     // This override replaces the base entirely, so fixing the base alone left
     // it waiting on `canplaythrough` forever.
     video.dispatchEvent(new Event('error'));
-    for (let i = 0; i < 6; i += 1) {
-      await settle();
-    }
+    await waitFor(() => log.codes.includes('figure-video.load-failed'));
 
-    expect(details.map((detail) => detail.code)).toContain('figure-video.load-failed');
     expect(getInstance<FigureVideoTwicpics>(el, 'FigureVideoTwicpics')!.hasLoaded).toBe(false);
 
-    document.removeEventListener('js-toolkit:diagnostic', listener);
+    log.stop();
   });
 });
 
diff --git a/packages/v4/migration/InView/InView.spec.ts b/packages/v4/migration/InView/InView.spec.ts
index 92c99af5c..09844a615 100644
--- a/packages/v4/migration/InView/InView.spec.ts
+++ b/packages/v4/migration/InView/InView.spec.ts
@@ -1,6 +1,5 @@
 import { afterEach, beforeEach, describe, expect, it } from 'vitest';
-import { Base, registerComponents, type BaseConfig } from '../../src/index.js';
-import { INSTANCES } from '../../src/protocol-symbols.js';
+import { Base, getInstance, registerComponents, type BaseConfig } from '../../src/index.js';
 import { recordEvents, resetDom, settle, waitFor } from '../../src/test/index.js';
 import { InView } from './InView.js';
 import { InViewOnce } from './InViewOnce.js';
@@ -79,7 +78,7 @@ describe('InView', () => {
   it('re-emits `in-view` on each re-entry, from the same instance', async () => {
     const el = render('InView', ONSCREEN);
     await waitFor(() => log.events.length > 0);
-    const instance = el[INSTANCES]?.get('InView');
+    const instance = getInstance(el, 'InView');
 
     el.setAttribute('style', OFFSCREEN);
     await waitFor(() => log.events.length > 1);
@@ -87,14 +86,14 @@ describe('InView', () => {
     await waitFor(() => log.events.length > 2);
 
     expect(types()).toEqual(['in-view', 'out-of-view', 'in-view']);
-    expect(el[INSTANCES]?.get('InView')).toBe(instance);
+    expect(getInstance(el, 'InView')).toBe(instance);
   });
 
   it('does not instantiate the component until it is first seen', async () => {
     const el = render('InView', OFFSCREEN);
     await quiet();
 
-    expect(el[INSTANCES]?.get('InView')).toBeUndefined();
+    expect(getInstance(el, 'InView')).toBeUndefined();
   });
 });
 
@@ -123,7 +122,7 @@ describe('InViewOnce', () => {
 
   it('stays mounted after leaving the viewport, where v3 terminated', async () => {
     const el = render('InViewOnce', ONSCREEN);
-    const instance = await waitFor(() => el[INSTANCES]?.get('InViewOnce'));
+    const instance = await waitFor(() => getInstance(el, 'InViewOnce'));
 
     el.setAttribute('style', OFFSCREEN);
     await quiet();
@@ -153,7 +152,7 @@ describe('mount strategy gaps found by the port', () => {
     await waitFor(() => log.events.length > 0);
 
     expect(types()).toEqual(['in-view']);
-    expect(el[INSTANCES]?.get('InView')?.$isMounted).toBe(true);
+    expect(getInstance(el, 'InView')?.$isMounted).toBe(true);
   });
 
   /** Subclasses must inherit the resolved mount strategy. */
@@ -161,7 +160,7 @@ describe('mount strategy gaps found by the port', () => {
     const el = render('InViewSubclass', OFFSCREEN);
     await quiet();
 
-    expect(el[INSTANCES]?.get('InViewSubclass')).toBeUndefined();
+    expect(getInstance(el, 'InViewSubclass')).toBeUndefined();
   });
 });
 
@@ -177,19 +176,19 @@ describe('the strategy is per element, which the decorator never was', () => {
 
   it('lets `data-mount` override the class default on one element', async () => {
     const el = render('InView', OFFSCREEN, { 'data-mount': 'eager' });
-    await waitFor(() => el[INSTANCES]?.get('InView')?.$isMounted);
+    await waitFor(() => getInstance(el, 'InView')?.$isMounted);
 
-    expect(el[INSTANCES]?.get('InView')?.$isMounted).toBe(true);
+    expect(getInstance(el, 'InView')?.$isMounted).toBe(true);
     expect(types()).toEqual(['in-view']);
   });
 
   it('lets `data-mount="in-view"` give the strategy to a component that never asked', async () => {
     const el = render('InViewEagerProbe', OFFSCREEN, { 'data-mount': 'in-view' });
     await quiet();
-    expect(el[INSTANCES]?.get('InViewEagerProbe')).toBeUndefined();
+    expect(getInstance(el, 'InViewEagerProbe')).toBeUndefined();
 
     el.setAttribute('style', ONSCREEN);
-    await waitFor(() => el[INSTANCES]?.get('InViewEagerProbe'));
-    expect((el[INSTANCES]?.get('InViewEagerProbe') as Eager | undefined)?.mounts).toBe(1);
+    await waitFor(() => getInstance(el, 'InViewEagerProbe'));
+    expect((getInstance(el, 'InViewEagerProbe') as Eager | undefined)?.mounts).toBe(1);
   });
 });
diff --git a/packages/v4/migration/Menu/Menu.spec.ts b/packages/v4/migration/Menu/Menu.spec.ts
index 521e5424d..c060fba1b 100644
--- a/packages/v4/migration/Menu/Menu.spec.ts
+++ b/packages/v4/migration/Menu/Menu.spec.ts
@@ -28,7 +28,7 @@ async function render(mode?: string): Promise<{ root: HTMLElement; menu: Menu }>
   root.innerHTML = menuMarkup(mode);
   document.body.append(root);
   await settle();
-  return { root, menu: getInstance<Menu>(root.querySelector('[data-component="Menu"]')!, 'Menu')! };
+  return { root, menu: getInstance<Menu>(root.querySelector('[data-component="Menu"]'), 'Menu')! };
 }
 
 describe('Menu', () => {
@@ -133,8 +133,8 @@ describe('Menu', () => {
       </div>`;
     document.body.append(root);
     await settle();
-    const subA = getInstance<Menu>(root.querySelector('#sub-a')!, 'Menu')!;
-    const subB = getInstance<Menu>(root.querySelector('#sub-b')!, 'Menu')!;
+    const subA = getInstance<Menu>(root.querySelector('#sub-a'), 'Menu')!;
+    const subB = getInstance<Menu>(root.querySelector('#sub-b'), 'Menu')!;
 
     subA.open();
     expect(subA.menuList?.isOpen).toBe(true);
diff --git a/packages/v4/migration/Menu/MenuList.spec.ts b/packages/v4/migration/Menu/MenuList.spec.ts
index f50a42b62..8c03861c6 100644
--- a/packages/v4/migration/Menu/MenuList.spec.ts
+++ b/packages/v4/migration/Menu/MenuList.spec.ts
@@ -25,8 +25,8 @@ async function render(): Promise<{
     </ul>`);
   return {
     root,
-    outer: getInstance<MenuList>(root.querySelector('#outer-list')!, 'MenuList')!,
-    nested: getInstance<MenuList>(root.querySelector('#nested-list')!, 'MenuList')!,
+    outer: getInstance<MenuList>(root.querySelector('#outer-list'), 'MenuList')!,
+    nested: getInstance<MenuList>(root.querySelector('#nested-list'), 'MenuList')!,
     outerLink: root.querySelector('#outer-link') as HTMLElement,
     nestedLink: root.querySelector('#nested-link') as HTMLElement,
   };
diff --git a/packages/v4/migration/Timer/TimerProgress.spec.ts b/packages/v4/migration/Timer/TimerProgress.spec.ts
index 5cc3b7e52..866a84dfb 100644
--- a/packages/v4/migration/Timer/TimerProgress.spec.ts
+++ b/packages/v4/migration/Timer/TimerProgress.spec.ts
@@ -1,6 +1,6 @@
 import { afterEach, describe, expect, it } from 'vitest';
 import { getInstance, registerComponents } from '../../src/index.js';
-import { resetDom, settle } from '../../src/test/index.js';
+import { recordEvents, resetDom, settle } from '../../src/test/index.js';
 import { TimerProgress } from './TimerProgress.js';
 
 registerComponents(TimerProgress);
@@ -20,12 +20,10 @@ async function render(attributes = ''): Promise<{ el: HTMLElement; instance: Tim
   return { el, instance: getInstance<TimerProgress>(el, 'TimerProgress')! };
 }
 
-function recordProgress(el: HTMLElement): number[] {
-  const ratios: number[] = [];
-  el.addEventListener('timer-progress', (event) => {
-    ratios.push((event as CustomEvent<{ ratio: number }>).detail.ratio);
-  });
-  return ratios;
+/** {@link recordEvents}, projected to the ratio each `timer-progress` carried. */
+function recordProgress(el: HTMLElement): () => number[] {
+  const log = recordEvents(el, 'timer-progress');
+  return () => log.events.map(({ detail }) => (detail as { ratio: number }).ratio);
 }
 
 describe('TimerProgress', () => {
@@ -35,7 +33,7 @@ describe('TimerProgress', () => {
 
     await wait(60);
 
-    expect(ratios).toEqual([]);
+    expect(ratios()).toEqual([]);
   });
 
   it('reports increasing progress while armed, ending at 1', async () => {
@@ -44,9 +42,9 @@ describe('TimerProgress', () => {
 
     await wait(150);
 
-    expect(ratios.length).toBeGreaterThan(1);
-    expect(ratios.at(-1)).toBe(1);
-    expect([...ratios]).toEqual([...ratios].sort((a, b) => a - b));
+    expect(ratios().length).toBeGreaterThan(1);
+    expect(ratios().at(-1)).toBe(1);
+    expect(ratios()).toEqual(ratios().sort((a, b) => a - b));
   });
 
   it('stops the frame loop once complete', async () => {
@@ -54,10 +52,10 @@ describe('TimerProgress', () => {
     const ratios = recordProgress(el);
 
     await wait(60);
-    const countAtComplete = ratios.length;
+    const countAtComplete = ratios().length;
     await wait(60);
 
-    expect(ratios.length).toBe(countAtComplete);
+    expect(ratios().length).toBe(countAtComplete);
   });
 
   it('resets progress to 0 when stopped', async () => {
@@ -67,7 +65,7 @@ describe('TimerProgress', () => {
     await wait(30);
     instance.stop();
 
-    expect(ratios.at(-1)).toBe(0);
+    expect(ratios().at(-1)).toBe(0);
   });
 
   it('stops the frame loop while paused and resumes it', async () => {
@@ -76,14 +74,14 @@ describe('TimerProgress', () => {
 
     await wait(20);
     instance.pause();
-    const countAtPause = ratios.length;
+    const countAtPause = ratios().length;
     await wait(40);
 
-    expect(ratios.length).toBe(countAtPause);
+    expect(ratios().length).toBe(countAtPause);
 
     instance.resume();
     await wait(150);
 
-    expect(ratios.at(-1)).toBe(1);
+    expect(ratios().at(-1)).toBe(1);
   });
 });
diff --git a/packages/v4/migration/Toaster/Toast.spec.ts b/packages/v4/migration/Toaster/Toast.spec.ts
index 011cf6fc7..0e8e49520 100644
--- a/packages/v4/migration/Toaster/Toast.spec.ts
+++ b/packages/v4/migration/Toaster/Toast.spec.ts
@@ -1,34 +1,12 @@
 import { afterEach, describe, expect, it } from 'vitest';
 import { getInstance, registerComponents } from '../../src/index.js';
-import { resetDom, settle } from '../../src/test/index.js';
+import { resetDom, settle, waitFor } from '../../src/test/index.js';
 import { Toast } from './Toast.js';
 
 registerComponents(Toast);
 
 afterEach(resetDom);
 
-function wait(ms: number): Promise<void> {
-  return new Promise((resolve) => setTimeout(resolve, ms));
-}
-
-/**
- * `viewTransition()` chains onto a module-level tail the scheduler does not
- * track: `defaultScheduler`'s write task that flushes it returns as soon as
- * `document.startViewTransition(...).finished` is *requested*, not once it
- * settles, so `settle()` gives no guarantee the DOM mutation inside it has
- * run yet. A real headless compositor can also take longer than usual to
- * finish one. Poll instead of trusting a fixed wait.
- */
-async function waitFor(predicate: () => boolean, timeout = 1000): Promise<void> {
-  const deadline = Date.now() + timeout;
-  while (!predicate()) {
-    if (Date.now() > deadline) {
-      throw new Error('waitFor: timed out');
-    }
-    await wait(10);
-  }
-}
-
 function renderUnmounted(attributes = ''): HTMLElement {
   const root = document.createElement('div');
   root.innerHTML = `
diff --git a/packages/v4/migration/Toaster/Toaster.spec.ts b/packages/v4/migration/Toaster/Toaster.spec.ts
index ed4de1c1c..522a93c03 100644
--- a/packages/v4/migration/Toaster/Toaster.spec.ts
+++ b/packages/v4/migration/Toaster/Toaster.spec.ts
@@ -1,6 +1,6 @@
 import { afterEach, describe, expect, it } from 'vitest';
 import { getInstance, registerComponents } from '../../src/index.js';
-import { resetDom, settle } from '../../src/test/index.js';
+import { resetDom, settle, waitFor } from '../../src/test/index.js';
 import { Toast } from './Toast.js';
 import { Toaster } from './Toaster.js';
 
@@ -8,22 +8,6 @@ registerComponents(Toaster, Toast);
 
 afterEach(resetDom);
 
-/**
- * `viewTransition()` chains onto a module-level tail the scheduler does not
- * track — `settle()` gives no guarantee the DOM mutation inside it has run —
- * and a real headless compositor can take longer than usual to finish one.
- * Poll instead of trusting a fixed wait.
- */
-async function waitFor(predicate: () => boolean, timeout = 1000): Promise<void> {
-  const deadline = Date.now() + timeout;
-  while (!predicate()) {
-    if (Date.now() > deadline) {
-      throw new Error('waitFor: timed out');
-    }
-    await new Promise((resolve) => setTimeout(resolve, 10));
-  }
-}
-
 async function render(): Promise<{ root: HTMLElement; instance: Toaster }> {
   const root = document.createElement('div');
   root.innerHTML = `
@@ -41,7 +25,7 @@ async function render(): Promise<{ root: HTMLElement; instance: Toaster }> {
   await settle();
   return {
     root,
-    instance: getInstance<Toaster>(root.querySelector('[data-component="Toaster"]')!, 'Toaster')!,
+    instance: getInstance<Toaster>(root.querySelector('[data-component="Toaster"]'), 'Toaster')!,
   };
 }
 
diff --git a/packages/v4/src/Base.spec.ts b/packages/v4/src/Base.spec.ts
index 175823b52..05e36634e 100644
--- a/packages/v4/src/Base.spec.ts
+++ b/packages/v4/src/Base.spec.ts
@@ -76,7 +76,7 @@ describe('$emit and delegation', () => {
     await settle();
 
     const li = root.querySelector('[data-component="TodoItem"]');
-    const instance = getInstance(li!, 'TodoItem')!;
+    const instance = getInstance(li, 'TodoItem')!;
     const seen: unknown[] = [];
     root.addEventListener('ping', (event) => {
       seen.push((event as CustomEvent).detail);
@@ -93,7 +93,7 @@ describe('$emit and delegation', () => {
     await settle();
 
     const li = root.querySelector('[data-component="TodoItem"]');
-    const instance = getInstance(li!, 'TodoItem')!;
+    const instance = getInstance(li, 'TodoItem')!;
     const seen: unknown[] = [];
     root.addEventListener('ping', (event) => seen.push((event as CustomEvent).detail));
 
@@ -106,7 +106,7 @@ describe('$emit and delegation', () => {
     await settle();
 
     const li = root.querySelector('[data-component="TodoItem"]');
-    const instance = getInstance(li!, 'TodoItem')!;
+    const instance = getInstance(li, 'TodoItem')!;
     const log = captureDiagnostics();
     const seen: unknown[] = [];
     root.addEventListener('ping', (event) => seen.push((event as CustomEvent).detail));
@@ -463,7 +463,6 @@ describe('$options', () => {
     const calls: string[] = [];
     const cleanupFailure = new Error('expected cleanup failure');
     const handlerFailure = new Error('expected handler failure');
-    const events: CustomEvent<ToolkitDiagnosticDetail>[] = [];
 
     class ResilientOptions extends Base {
       static config = {
@@ -501,10 +500,7 @@ describe('$options', () => {
       registerComponent(ReentrantOption);
       const resilient = document.createElement('div');
       resilient.setAttribute('data-component', 'ResilientOptions');
-      resilient.addEventListener(EVENTS.diagnostic, (event) => {
-        event.preventDefault();
-        events.push(event as CustomEvent<ToolkitDiagnosticDetail>);
-      });
+      const log = captureDiagnostics(resilient);
       const reentrant = document.createElement('div');
       reentrant.setAttribute('data-component', 'ReentrantOption');
       document.body.append(resilient, reentrant);
@@ -516,12 +512,13 @@ describe('$options', () => {
       await settle();
 
       expect(calls).toEqual(['second:0', 'second:2']);
-      expect(events.map((event) => event.detail.error)).toEqual([cleanupFailure, handlerFailure]);
-      expect(
-        events.every((event) => event.detail.code === DIAGNOSTICS.component.lifecycleFailed),
-      ).toBe(true);
-      expect(events.every((event) => event.detail.component === 'ResilientOptions')).toBe(true);
+      expect(log.entries.map(({ error }) => error)).toEqual([cleanupFailure, handlerFailure]);
+      expect(log.entries.every(({ code }) => code === DIAGNOSTICS.component.lifecycleFailed)).toBe(
+        true,
+      );
+      expect(log.entries.every(({ component }) => component === 'ResilientOptions')).toBe(true);
       expect(getInstance<ReentrantOption>(reentrant, 'ReentrantOption')!.$isMounted).toBe(false);
+      log.stop();
     } finally {
       document.querySelectorAll('[data-component="ResilientOptions"]').forEach((el) => el.remove());
     }
@@ -797,7 +794,7 @@ describe('$refs', () => {
     expect(owner.$refs.item).toBe(root.querySelector('[data-ref="item"]'));
 
     await settle();
-    expect(getInstance(root.lastElementChild!, 'RefReadInserted')!.$isMounted).toBe(true);
+    expect(getInstance(root.lastElementChild, 'RefReadInserted')!.$isMounted).toBe(true);
   });
 });
 
@@ -1330,6 +1327,8 @@ describe('$watchChildren', () => {
     for (const instance of [unrelated, gamma, family, beta, alpha]) {
       instance.$mount();
     }
+    // A raw write, so no lookup replaces it: the fixture needs one instance
+    // filed under a second name, and `getInstance()` only reads.
     alpha.$el[INSTANCES]?.set('WatchAlphaAlias', alpha);
 
     const owner = new Owner(root);
@@ -1618,7 +1617,7 @@ describe('lifecycle', () => {
     expect(instance.$isMounted).toBe(false);
     // Unmount is reversible, so the instance stays on its element for a later
     // mount even when both its hooks threw.
-    expect(el[INSTANCES]?.get('TeardownFailure')).toBe(instance);
+    expect(getInstance(el, 'TeardownFailure')).toBe(instance);
   });
 
   it('runs the mounted() cleanup on unmount', async () => {
@@ -1626,7 +1625,7 @@ describe('lifecycle', () => {
     await settle();
 
     const countInstance = getInstance<TodoCount>(
-      root.querySelector('[data-component="TodoCount"]')!,
+      root.querySelector('[data-component="TodoCount"]'),
       'TodoCount',
     )!;
     expect(countInstance.cleanupCalls).toBe(0);
@@ -1661,14 +1660,14 @@ describe('lifecycle', () => {
 
     instance.$mount();
     expect(instance.$isMounted).toBe(true);
-    expect(el[INSTANCES]?.get('Tracked')).toBe(instance);
+    expect(getInstance(el, 'Tracked')).toBe(instance);
 
     // Mount and unmount are the whole lifecycle: neither is one-way, and the
     // instance stays on its element between them, which is what lets a moved
     // or re-inserted element keep its identity.
     instance.$unmount();
     expect(calls).toEqual(['mounted', 'cleanup', 'unmounted', 'mounted', 'cleanup', 'unmounted']);
-    expect(el[INSTANCES]?.get('Tracked')).toBe(instance);
+    expect(getInstance(el, 'Tracked')).toBe(instance);
 
     instance.$mount();
     expect(instance.$isMounted).toBe(true);
diff --git a/packages/v4/src/attribute-namespaces.spec.ts b/packages/v4/src/attribute-namespaces.spec.ts
index 40aa214b6..f33a40de0 100644
--- a/packages/v4/src/attribute-namespaces.spec.ts
+++ b/packages/v4/src/attribute-namespaces.spec.ts
@@ -1,8 +1,6 @@
 import { afterEach, describe, expect, it, vi } from 'vitest';
 import { watchAttributeNamespace } from './attribute-namespaces.js';
-import { EVENTS } from './events.js';
-import { resetDom, settle } from './test/index.js';
-import type { ToolkitDiagnosticDetail } from './diagnostic-contract.js';
+import { captureDiagnostics, resetDom, settle } from './test/index.js';
 
 const cleanups = new Set<() => void>();
 
@@ -158,11 +156,7 @@ describe('watchAttributeNamespace', () => {
   describe('a declared vocabulary', () => {
     it('binds a known head and warns once for an unknown one', async () => {
       const el = element('<div data-bind:text="a" data-bind:prpo.value="b"></div>');
-      const details: ToolkitDiagnosticDetail[] = [];
-      document.addEventListener(EVENTS.diagnostic, (event) => {
-        details.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-        event.preventDefault();
-      });
+      const log = captureDiagnostics();
       const record = recorder();
       watched(el, 'data-bind', record.bind, {
         qualifiers: ['text', 'if', 'prop'],
@@ -170,16 +164,17 @@ describe('watchAttributeNamespace', () => {
       });
 
       expect(record.bound).toEqual(['data-bind:text|text|a']);
-      expect(details).toHaveLength(1);
-      expect(details[0].code).toBe('attribute.unknown-qualifier');
-      expect(details[0].severity).toBe('warning');
-      expect(details[0].component).toBe('DataBind');
-      expect(details[0].message).toContain('prpo');
+      expect(log.entries).toHaveLength(1);
+      expect(log.entries[0].code).toBe('attribute.unknown-qualifier');
+      expect(log.entries[0].severity).toBe('warning');
+      expect(log.entries[0].component).toBe('DataBind');
+      expect(log.entries[0].message).toContain('prpo');
 
       // Once per element and per name, whatever the value is rewritten to.
       el.setAttribute('data-bind:prpo.value', 'c');
       await settle();
-      expect(details).toHaveLength(1);
+      expect(log.entries).toHaveLength(1);
+      log.stop();
     });
 
     it('validates the head only, so the name after the dot stays open', () => {
@@ -209,22 +204,19 @@ describe('watchAttributeNamespace', () => {
     // the second separator itself — nothing else stands between this and
     // `Action` binding a literal `click:s` DOM event.
     const el = element('<div data-on:click:s="a"></div>');
-    const details: ToolkitDiagnosticDetail[] = [];
-    document.addEventListener(EVENTS.diagnostic, (event) => {
-      details.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-      event.preventDefault();
-    });
+    const log = captureDiagnostics();
     const record = recorder();
     watched(el, 'data-on', record.bind);
 
     expect(record.bound).toEqual([]);
-    expect(details).toHaveLength(1);
-    expect(details[0].code).toBe('attribute.unknown-qualifier');
+    expect(log.entries).toHaveLength(1);
+    expect(log.entries[0].code).toBe('attribute.unknown-qualifier');
 
     // Once per element and per name, not re-triggered by a rewrite of the value.
     el.setAttribute('data-on:click:s', 'b');
     await settle();
     expect(record.bound).toEqual([]);
-    expect(details).toHaveLength(1);
+    expect(log.entries).toHaveLength(1);
+    log.stop();
   });
 });
diff --git a/packages/v4/src/autoload.spec.ts b/packages/v4/src/autoload.spec.ts
index 9c0f61bb0..2d019dad8 100644
--- a/packages/v4/src/autoload.spec.ts
+++ b/packages/v4/src/autoload.spec.ts
@@ -3,8 +3,7 @@ import { Base, type BaseConfig, type BaseConstructor } from './Base.js';
 import { DIAGNOSTICS, type ToolkitDiagnosticDetail } from './diagnostic-contract.js';
 import { whenDOMSettled } from './dom-mutations.js';
 import { EVENTS } from './events.js';
-import { getInstances } from './instances.js';
-import { INSTANCES } from './protocol-symbols.js';
+import { getInstance, getInstances } from './instances.js';
 import { registerComponent, registerManifest } from './registry.js';
 import { captureDiagnostics, resetDom, settle, waitFor } from './test/index.js';
 
@@ -64,10 +63,6 @@ function render(name: string, attributes: Record<string, string> = {}, style = O
   return el;
 }
 
-function instanceOf<T extends Base>(el: Element, name: string): T | undefined {
-  return el[INSTANCES]?.get(name) as T | undefined;
-}
-
 /**
  * A bounded quiet period. Waiting for something to arrive is a poll — see the
  * `waitFor` calls below — but an assertion that nothing was imported cannot be
@@ -87,13 +82,13 @@ describe('registerManifest', () => {
     const el = render(name);
     await settle();
 
-    expect(el[INSTANCES]).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
 
     registerManifest({ [name]: load });
     await settle();
 
     expect(importCount()).toBe(1);
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 
   it('imports and mounts a component inserted after the manifest', async () => {
@@ -104,7 +99,7 @@ describe('registerManifest', () => {
     const el = render(name);
     await settle();
 
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 
   it('imports the module once for every element declaring the token', async () => {
@@ -115,8 +110,8 @@ describe('registerManifest', () => {
     await settle();
 
     expect(importCount()).toBe(1);
-    expect(instanceOf(first, name)?.$isMounted).toBe(true);
-    expect(instanceOf(second, name)?.$isMounted).toBe(true);
+    expect(getInstance(first, name)?.$isMounted).toBe(true);
+    expect(getInstance(second, name)?.$isMounted).toBe(true);
   });
 
   it('imports nothing when no element declares the token', async () => {
@@ -139,8 +134,8 @@ describe('registerManifest', () => {
     });
     await settle();
 
-    expect(instanceOf(namedEl, named.name)?.$isMounted).toBe(true);
-    expect(instanceOf(defaultEl, fallback.name)?.$isMounted).toBe(true);
+    expect(getInstance(namedEl, named.name)?.$isMounted).toBe(true);
+    expect(getInstance(defaultEl, fallback.name)?.$isMounted).toBe(true);
   });
 });
 
@@ -151,14 +146,14 @@ describe('a lazy declaration before its class arrives', () => {
     registerManifest({ [name]: { load, mountStrategy: 'visible' } });
     await quiet();
 
-    expect(el[INSTANCES]).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
     expect(getInstances(name)).toEqual([]);
 
     el.setAttribute('style', ONSCREEN);
     await waitFor(() => getInstances(name).length > 0);
 
     expect(getInstances(name)).toHaveLength(1);
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 
   it('is dropped when its token is removed before the trigger fires', async () => {
@@ -191,7 +186,7 @@ describe('a lazy declaration before its class arrives', () => {
     await waitFor(() => importCount() > 0);
 
     expect(importCount()).toBe(1);
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 });
 
@@ -208,7 +203,7 @@ describe('the strategy that triggers the import', () => {
     await waitFor(() => importCount() > 0);
 
     expect(importCount()).toBe(1);
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 
   it('lets the element data-mount win over the entry default', async () => {
@@ -224,7 +219,7 @@ describe('the strategy that triggers the import', () => {
     await settle();
 
     expect(importCount()).toBe(1);
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 
   // An invalid strategy is reported and kept inert by the one scheduling path
@@ -232,34 +227,31 @@ describe('the strategy that triggers the import', () => {
   // is that nothing is downloaded for a trigger which never fires.
   it('imports nothing for an invalid trigger, and imports when data-mount is corrected', async () => {
     const { name, load, importCount } = defineLazy();
-    const diagnostics: ToolkitDiagnosticDetail[] = [];
     const el = render(name, { 'data-mount': 'eagre' });
-    el.addEventListener(EVENTS.diagnostic, (event) => {
-      event.preventDefault();
-      diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-    });
+    const log = captureDiagnostics(el);
     registerManifest({ [name]: load });
-    await waitFor(() => diagnostics.length > 0);
+    await waitFor(() => log.entries.length > 0);
 
     expect(importCount()).toBe(0);
-    expect(el[INSTANCES]?.get(name)).toBeUndefined();
-    expect(diagnostics).toHaveLength(1);
-    expect(diagnostics[0].code).toBe(DIAGNOSTICS.component.invalidMountStrategy);
+    expect(getInstance(el, name)).toBeUndefined();
+    expect(log.entries).toHaveLength(1);
+    expect(log.entries[0].code).toBe(DIAGNOSTICS.component.invalidMountStrategy);
+    log.stop();
 
     el.setAttribute('data-mount', 'eager');
     await settle();
     expect(importCount()).toBe(1);
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 
   it('imports on a media query which matches while its trigger is being applied', async () => {
     const { name, load, importCount } = defineLazy();
     const el = render(name);
     registerManifest({ [name]: { load, mountStrategy: 'media:(min-width: 1px)' } });
-    await waitFor(() => instanceOf(el, name)?.$isMounted);
+    await waitFor(() => getInstance(el, name)?.$isMounted);
 
     expect(importCount()).toBe(1);
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 
   it('imports nothing while the media query does not match', async () => {
@@ -269,7 +261,7 @@ describe('the strategy that triggers the import', () => {
     await quiet();
 
     expect(importCount()).toBe(0);
-    expect(el[INSTANCES]).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
   });
 
   it('hands a parameterized reversible override to the registry after the import', async () => {
@@ -281,7 +273,7 @@ describe('the strategy that triggers the import', () => {
     expect(importCount()).toBe(0);
 
     el.setAttribute('style', ONSCREEN);
-    const instance = await waitFor(() => instanceOf(el, name));
+    const instance = await waitFor(() => getInstance(el, name));
 
     expect(importCount()).toBe(1);
     expect(instance?.$isMounted).toBe(true);
@@ -321,12 +313,12 @@ describe('the strategy that triggers the import', () => {
     release();
     await quiet();
 
-    expect(el[INSTANCES]?.get(name)).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
 
     el.setAttribute('style', ONSCREEN);
-    await waitFor(() => instanceOf(el, name)?.$isMounted);
+    await waitFor(() => getInstance(el, name)?.$isMounted);
 
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 });
 
@@ -339,7 +331,7 @@ describe('whenDOMSettled and a lazy component', () => {
     const el = render(name);
     await whenDOMSettled();
 
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 
   it('does not wait for a conditional trigger', async () => {
@@ -378,7 +370,7 @@ describe('registerManifest collisions and failures', () => {
         message: `"${name}" is already registered; the incoming declaration was ignored.`,
       },
     ]);
-    expect(instanceOf(el, name)).toBeInstanceOf(Owned);
+    expect(getInstance(el, name)).toBeInstanceOf(Owned);
     log.stop();
   });
 
@@ -446,27 +438,22 @@ describe('registerManifest collisions and failures', () => {
       component: name,
     });
     expect(elementEvents[0].detail.error).toBe(failure);
-    expect(first[INSTANCES]).toBeUndefined();
-    expect(second[INSTANCES]).toBeUndefined();
+    expect(getInstance(first, name)).toBeUndefined();
+    expect(getInstance(second, name)).toBeUndefined();
   });
 
   it('reports a module which resolves to no component class', async () => {
     counter += 1;
     const name = `Empty${counter}`;
-    const diagnostics: ToolkitDiagnosticDetail[] = [];
-    const onDiagnostic = (event: Event) => {
-      event.preventDefault();
-      diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-    };
-    document.addEventListener(EVENTS.diagnostic, onDiagnostic);
+    const log = captureDiagnostics();
 
     registerManifest({ [name]: async () => ({ notAClass: 42 }) });
     render(name);
     await settle();
-    document.removeEventListener(EVENTS.diagnostic, onDiagnostic);
+    log.stop();
 
-    expect(diagnostics).toHaveLength(1);
-    expect(diagnostics[0]).toMatchObject({
+    expect(log.entries).toHaveLength(1);
+    expect(log.entries[0]).toMatchObject({
       severity: 'error',
       code: DIAGNOSTICS.component.loadFailed,
       component: name,
@@ -490,7 +477,7 @@ describe('registerManifest collisions and failures', () => {
         message: `"${token}" resolved to a component named "${name}".`,
       },
     ]);
-    expect(el[INSTANCES]).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
     log.stop();
   });
 });
@@ -509,7 +496,7 @@ describe('a dynamic import declared in config.components', () => {
     await settle();
 
     expect(child.importCount()).toBe(1);
-    expect(instanceOf(el, child.name)?.$isMounted).toBe(true);
+    expect(getInstance(el, child.name)?.$isMounted).toBe(true);
   });
 
   it('registers the class half of a mixed map right away', async () => {
@@ -527,14 +514,14 @@ describe('a dynamic import declared in config.components', () => {
     const siblingEl = render(eagerName);
     await settle();
 
-    expect(instanceOf(siblingEl, eagerName)).toBeInstanceOf(Sibling);
+    expect(getInstance(siblingEl, eagerName)).toBeInstanceOf(Sibling);
     expect(lazy.importCount()).toBe(0);
 
     const lazyEl = render(lazy.name);
     await settle();
 
     expect(lazy.importCount()).toBe(1);
-    expect(instanceOf(lazyEl, lazy.name)?.$isMounted).toBe(true);
+    expect(getInstance(lazyEl, lazy.name)?.$isMounted).toBe(true);
   });
 
   it('imports the module once for every element declaring the child', async () => {
@@ -547,8 +534,8 @@ describe('a dynamic import declared in config.components', () => {
     await settle();
 
     expect(child.importCount()).toBe(1);
-    expect(instanceOf(first, child.name)?.$isMounted).toBe(true);
-    expect(instanceOf(second, child.name)?.$isMounted).toBe(true);
+    expect(getInstance(first, child.name)?.$isMounted).toBe(true);
+    expect(getInstance(second, child.name)?.$isMounted).toBe(true);
   });
 
   it('imports once when two parents declare the same child', async () => {
@@ -588,8 +575,8 @@ describe('a dynamic import declared in config.components', () => {
 
     expect(parentImports).toBe(1);
     expect(child.importCount()).toBe(1);
-    expect(instanceOf(root, parentName)?.$isMounted).toBe(true);
-    expect(instanceOf(childEl, child.name)?.$isMounted).toBe(true);
+    expect(getInstance(root, parentName)?.$isMounted).toBe(true);
+    expect(getInstance(childEl, child.name)?.$isMounted).toBe(true);
   });
 
   it('honours the element data-mount of a child nobody has imported', async () => {
@@ -606,7 +593,7 @@ describe('a dynamic import declared in config.components', () => {
     await waitFor(() => child.importCount() > 0);
 
     expect(child.importCount()).toBe(1);
-    expect(instanceOf(el, child.name)?.$isMounted).toBe(true);
+    expect(getInstance(el, child.name)?.$isMounted).toBe(true);
   });
 
   it('reports a value which is neither a class nor an importer', async () => {
@@ -659,7 +646,7 @@ describe('the family a subclass inherits', () => {
     const el = render(childName);
     await settle();
 
-    expect(instanceOf(el, childName)).toBeInstanceOf(Child);
+    expect(getInstance(el, childName)).toBeInstanceOf(Child);
   });
 
   it('registers a lazy child its base declared', async () => {
@@ -672,7 +659,7 @@ describe('the family a subclass inherits', () => {
     await settle();
 
     expect(child.importCount()).toBe(1);
-    expect(instanceOf(el, child.name)?.$isMounted).toBe(true);
+    expect(getInstance(el, child.name)?.$isMounted).toBe(true);
   });
 
   it('lets a subclass override one key without dropping the rest', async () => {
@@ -688,9 +675,9 @@ describe('the family a subclass inherits', () => {
     await settle();
 
     expect(stale).not.toHaveBeenCalled();
-    expect(instanceOf(overriddenEl, overridden.name)).toBeInstanceOf(overridden.Lazy);
+    expect(getInstance(overriddenEl, overridden.name)).toBeInstanceOf(overridden.Lazy);
     expect(kept.importCount()).toBe(1);
-    expect(instanceOf(keptEl, kept.name)?.$isMounted).toBe(true);
+    expect(getInstance(keptEl, kept.name)?.$isMounted).toBe(true);
   });
 
   it('registers a shared family once when the base registers too', async () => {
@@ -706,7 +693,7 @@ describe('the family a subclass inherits', () => {
 
     expect(log.codes).toEqual([]);
     expect(child.importCount()).toBe(1);
-    expect(instanceOf(el, child.name)?.$isMounted).toBe(true);
+    expect(getInstance(el, child.name)?.$isMounted).toBe(true);
     log.stop();
   });
 });
diff --git a/packages/v4/src/context-subscription.spec.ts b/packages/v4/src/context-subscription.spec.ts
index 0110518a9..60210b702 100644
--- a/packages/v4/src/context-subscription.spec.ts
+++ b/packages/v4/src/context-subscription.spec.ts
@@ -2,11 +2,10 @@ import { afterEach, describe, expect, it } from 'vitest';
 import { Base } from './Base.js';
 import { subscribeContext } from './context-subscription.js';
 import { createContext, provideContext, provideRootContext, type ContextKey } from './context.js';
-import { DIAGNOSTICS, type ToolkitDiagnosticDetail } from './diagnostic-contract.js';
-import { EVENTS } from './events.js';
+import { DIAGNOSTICS } from './diagnostic-contract.js';
 import { getInstance } from './instances.js';
 import { registerComponent } from './registry.js';
-import { resetDom, settle } from './test/index.js';
+import { captureDiagnostics, resetDom, settle } from './test/index.js';
 
 afterEach(resetDom);
 
@@ -130,7 +129,7 @@ describe('subscribeContext', () => {
     `);
     await settle();
 
-    const member = getInstance<Member>(root.querySelector('span')!, 'ReanswerMember')!;
+    const member = getInstance<Member>(root.querySelector('span'), 'ReanswerMember')!;
     expect(member.seen).toEqual(['page']);
 
     root.querySelector('#scope')?.setAttribute('data-component', 'ReanswerScope');
@@ -165,7 +164,7 @@ describe('subscribeContext', () => {
     `);
     await settle();
 
-    const member = getInstance<Member>(root.querySelector('span')!, 'DistanceMember')!;
+    const member = getInstance<Member>(root.querySelector('span'), 'DistanceMember')!;
     expect(member.seen).toEqual(['inner']);
 
     root.querySelector('#outer')?.setAttribute('data-component', 'DistanceScope');
@@ -235,15 +234,11 @@ describe('subscribeContext', () => {
   });
 
   it('isolates callback and teardown failures', () => {
-    const diagnostics: ToolkitDiagnosticDetail[] = [];
     const key = createContext<Registry>('failure-isolation');
     const root = render('<span></span>');
     const consumer = root.querySelector('span') as Element;
     provideContext(root, key, { name: 'root' });
-    consumer.addEventListener(EVENTS.diagnostic, (event) => {
-      event.preventDefault();
-      diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-    });
+    const log = captureDiagnostics(consumer);
 
     const failed = subscribeContext(consumer, key, () => {
       throw new Error('callback');
@@ -259,11 +254,12 @@ describe('subscribeContext', () => {
     failed();
     teardownFailed();
     expect(seen).toEqual(['root']);
-    expect(diagnostics.map(({ code }) => code)).toEqual([
+    expect(log.codes).toEqual([
       DIAGNOSTICS.callback.contextSubscriptionFailed,
       DIAGNOSTICS.callback.contextTeardownFailed,
     ]);
-    expect(diagnostics.every(({ severity }) => severity === 'error')).toBe(true);
+    expect(log.entries.every(({ severity }) => severity === 'error')).toBe(true);
+    log.stop();
   });
 
   it('does not keep a discarded consumer alive after it was answered', async () => {
diff --git a/packages/v4/src/context.spec.ts b/packages/v4/src/context.spec.ts
index de1a55d9f..8ffd757f6 100644
--- a/packages/v4/src/context.spec.ts
+++ b/packages/v4/src/context.spec.ts
@@ -9,12 +9,11 @@ import {
   signal,
   type Signal,
 } from './context.js';
-import { DIAGNOSTICS, type ToolkitDiagnosticDetail } from './diagnostic-contract.js';
-import { EVENTS } from './events.js';
+import { DIAGNOSTICS } from './diagnostic-contract.js';
 import { getInstance } from './instances.js';
 import { registerComponent } from './registry.js';
 import { renderTodoList } from './todo.fixtures.js';
-import { resetDom, settle } from './test/index.js';
+import { captureDiagnostics, resetDom, settle } from './test/index.js';
 
 afterEach(resetDom);
 
@@ -60,15 +59,7 @@ describe('Signal', () => {
   it('returns a usable unsubscribe after an immediate subscriber failure', () => {
     const cell = signal(0);
     const failure = new Error('immediate signal failure');
-    const diagnostics: ToolkitDiagnosticDetail[] = [];
-    document.addEventListener(
-      EVENTS.diagnostic,
-      (event) => {
-        event.preventDefault();
-        diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-      },
-      { once: true },
-    );
+    const log = captureDiagnostics();
 
     const unsubscribe = cell.subscribe(
       () => {
@@ -82,7 +73,7 @@ describe('Signal', () => {
     cell.value = 1;
 
     expect(seen).toEqual([1]);
-    expect(diagnostics).toEqual([
+    expect(log.entries).toEqual([
       {
         severity: 'error',
         code: DIAGNOSTICS.callback.signalFailed,
@@ -90,17 +81,13 @@ describe('Signal', () => {
         error: failure,
       },
     ]);
+    log.stop();
   });
 
   it('isolates update failures without staling a reentrant delivery round', () => {
     const cell = signal(0);
     const failure = new Error('signal update failure');
-    const diagnostics: ToolkitDiagnosticDetail[] = [];
-    const listener = (event: Event) => {
-      event.preventDefault();
-      diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-    };
-    document.addEventListener(EVENTS.diagnostic, listener);
+    const log = captureDiagnostics();
     const broken: number[] = [];
     const later: number[] = [];
     cell.subscribe((value) => {
@@ -113,15 +100,15 @@ describe('Signal', () => {
     cell.subscribe((value) => later.push(value));
 
     cell.value = 1;
-    document.removeEventListener(EVENTS.diagnostic, listener);
+    log.stop();
 
     expect(cell.value).toBe(2);
     expect(broken).toEqual([1, 2]);
     expect(later).toEqual([2]);
-    expect(diagnostics).toHaveLength(2);
-    expect(diagnostics.every(({ code }) => code === DIAGNOSTICS.callback.signalFailed)).toBe(true);
+    expect(log.entries).toHaveLength(2);
+    expect(log.entries.every(({ code }) => code === DIAGNOSTICS.callback.signalFailed)).toBe(true);
     expect(
-      diagnostics.every((detail) => detail.severity === 'error' && detail.error === failure),
+      log.entries.every((detail) => detail.severity === 'error' && detail.error === failure),
     ).toBe(true);
   });
 
@@ -445,7 +432,7 @@ describe('provide/inject', () => {
 
     const counter = getInstance<Counter>(root, 'Counter')!;
     const button = root.querySelector('button');
-    const control = getInstance<CounterBtn>(button!, 'CounterBtn')!;
+    const control = getInstance<CounterBtn>(button, 'CounterBtn')!;
 
     button?.click();
     button?.click();
diff --git a/packages/v4/src/decorators.spec.ts b/packages/v4/src/decorators.spec.ts
index 7eca02cb2..5724f3361 100644
--- a/packages/v4/src/decorators.spec.ts
+++ b/packages/v4/src/decorators.spec.ts
@@ -644,7 +644,7 @@ describe('@on', () => {
 
     const parent = getInstance<DecoParent>(root, 'DecoParent')!;
     const child = getInstance<DecoChild>(
-      root.querySelector('[data-component="DecoChild"]')!,
+      root.querySelector('[data-component="DecoChild"]'),
       'DecoChild',
     )!;
 
@@ -660,7 +660,7 @@ describe('@on', () => {
 
     const parent = getInstance<DecoParent>(root, 'DecoParent')!;
     const child = getInstance<DecoChild>(
-      root.querySelector('[data-component="DecoChild"]')!,
+      root.querySelector('[data-component="DecoChild"]'),
       'DecoChild',
     )!;
 
@@ -718,7 +718,7 @@ describe('@on', () => {
 
     const parent = getInstance<DecoParent>(root, 'DecoParent')!;
     const child = getInstance<DecoChild>(
-      root.querySelector('[data-component="DecoChild"]')!,
+      root.querySelector('[data-component="DecoChild"]'),
       'DecoChild',
     )!;
 
@@ -849,9 +849,9 @@ describe('@on', () => {
     await settle();
 
     const parent = getInstance<SubTargetParent>(root, 'SubTargetParent')!;
-    const sub = getInstance<SubKind>(root.querySelector('[data-component="SubKind"]')!, 'SubKind')!;
+    const sub = getInstance<SubKind>(root.querySelector('[data-component="SubKind"]'), 'SubKind')!;
     const base = getInstance<BaseKind>(
-      root.querySelector('[data-component="BaseKind"]')!,
+      root.querySelector('[data-component="BaseKind"]'),
       'BaseKind',
     )!;
 
@@ -1043,7 +1043,7 @@ describe('@on stacked with @read / @write', () => {
 
     const instance = getInstance<PhasedHandlers>(root, 'PhasedHandlers')!;
     const child = getInstance<PhasedChild>(
-      root.querySelector('[data-component="PhasedChild"]')!,
+      root.querySelector('[data-component="PhasedChild"]'),
       'PhasedChild',
     )!;
 
@@ -1101,7 +1101,7 @@ describe('@provide / @inject', () => {
 
     const parent = getInstance<DecoParent>(root, 'DecoParent')!;
     const child = getInstance<DecoChild>(
-      root.querySelector('[data-component="DecoChild"]')!,
+      root.querySelector('[data-component="DecoChild"]'),
       'DecoChild',
     )!;
 
diff --git a/packages/v4/src/diagnostics.spec.ts b/packages/v4/src/diagnostics.spec.ts
index c0203e19f..c0632d0c9 100644
--- a/packages/v4/src/diagnostics.spec.ts
+++ b/packages/v4/src/diagnostics.spec.ts
@@ -8,7 +8,17 @@ import {
 } from './diagnostic-contract.js';
 import { reportDiagnostic, warn, warnOnce } from './diagnostics.js';
 import { EVENTS } from './events.js';
+import { captureDiagnostics } from './test/index.js';
 
+/**
+ * The listeners below are hand-rolled on purpose: this spec owns the channel
+ * that `captureDiagnostics()` reads, so the helper cannot instrument it.
+ * Every remaining raw listener asserts something the helper hides —
+ * cancellation itself, the order against the default sink, the target the
+ * dispatch started on, or what a listener mutating the detail does — and
+ * `captureDiagnostics()` cancels every event and exposes no target, which
+ * would make each of them vacuous.
+ */
 describe('diagnostics', () => {
   it('exposes exact deeply frozen stable codes', () => {
     expect(DIAGNOSTICS).toEqual({
@@ -257,19 +267,15 @@ describe('diagnostics', () => {
   it('deduplicates by weak owner and misuse key, not by message', () => {
     const firstOwner = {};
     const secondOwner = {};
-    const events: ToolkitDiagnosticDetail[] = [];
-    const listener = (rawEvent: Event) => {
-      rawEvent.preventDefault();
-      events.push((rawEvent as CustomEvent<ToolkitDiagnosticDetail>).detail);
-    };
-    document.addEventListener(EVENTS.diagnostic, listener);
+    // The one test here that only counts dispatches, so the helper fits.
+    const log = captureDiagnostics();
 
     warnOnce(firstOwner, 'a', DIAGNOSTICS.registry.conflict, 'Same message.');
     warnOnce(firstOwner, 'a', DIAGNOSTICS.registry.conflict, 'Same message.');
     warnOnce(firstOwner, 'b', DIAGNOSTICS.registry.conflict, 'Same message.');
     warnOnce(secondOwner, 'a', DIAGNOSTICS.registry.conflict, 'Same message.');
 
-    document.removeEventListener(EVENTS.diagnostic, listener);
-    expect(events).toHaveLength(3);
+    log.stop();
+    expect(log.entries).toHaveLength(3);
   });
 });
diff --git a/packages/v4/src/dom-mutations.spec.ts b/packages/v4/src/dom-mutations.spec.ts
index 66971a1af..13329853e 100644
--- a/packages/v4/src/dom-mutations.spec.ts
+++ b/packages/v4/src/dom-mutations.spec.ts
@@ -1,7 +1,7 @@
 import { afterEach, describe, expect, it, vi } from 'vitest';
 import { FRAMEWORK_ATTRIBUTES } from './attributes.js';
 import { Base, type BaseConfig, type OptionChange } from './Base.js';
-import { DIAGNOSTICS, type ToolkitDiagnosticDetail } from './diagnostic-contract.js';
+import { DIAGNOSTICS } from './diagnostic-contract.js';
 import {
   isObservedDOMAttribute,
   registerDOMOptionAttributes,
@@ -11,12 +11,10 @@ import {
   whenDOMSettled,
   type AttributeChange,
 } from './dom-mutations.js';
-import { EVENTS } from './events.js';
 import { getInstance } from './instances.js';
-import { INSTANCES } from './protocol-symbols.js';
 import { registerComponent } from './registry.js';
 import { SWAP_MODES, swap } from './swap.js';
-import { resetDom } from './test/index.js';
+import { captureDiagnostics, resetDom } from './test/index.js';
 
 let counter = 0;
 
@@ -55,7 +53,7 @@ describe('whenDOMSettled', () => {
     document.body.append(el);
 
     await whenDOMSettled();
-    expect(el[INSTANCES]?.get(name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 
   it('follows mutations created by eager lifecycle work', async () => {
@@ -83,7 +81,7 @@ describe('whenDOMSettled', () => {
 
     await whenDOMSettled();
     const child = parent.firstElementChild;
-    expect(child?.[INSTANCES]?.get(childName)?.$isMounted).toBe(true);
+    expect(getInstance(child, childName)?.$isMounted).toBe(true);
   });
 
   it('does not wait for a conditional mount strategy', async () => {
@@ -100,7 +98,7 @@ describe('whenDOMSettled', () => {
     document.body.append(el);
 
     await whenDOMSettled();
-    expect(el[INSTANCES]?.get(name)).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
   });
 
   it('waits for eager teardown caused by removal', async () => {
@@ -118,7 +116,7 @@ describe('whenDOMSettled', () => {
     el.setAttribute('data-component', name);
     document.body.append(el);
     await whenDOMSettled();
-    const instance = el[INSTANCES]?.get(name) as Removed;
+    const instance = getInstance(el, name) as Removed;
 
     el.remove();
     await whenDOMSettled();
@@ -230,11 +228,7 @@ describe('watchAttributes', () => {
     const el = document.createElement('div');
     document.body.append(el);
     const failure = new Error('watcher failure');
-    const diagnostics: ToolkitDiagnosticDetail[] = [];
-    el.addEventListener(EVENTS.diagnostic, (event) => {
-      event.preventDefault();
-      diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-    });
+    const log = captureDiagnostics(el);
     const changes: AttributeChange[] = [];
     trackedWatcher(el, () => {
       throw failure;
@@ -244,7 +238,7 @@ describe('watchAttributes', () => {
     el.setAttribute(VIRTUAL_ATTRIBUTE, 'open()');
     await whenDOMSettled();
 
-    expect(diagnostics).toEqual([
+    expect(log.entries).toEqual([
       {
         severity: 'error',
         code: DIAGNOSTICS.callback.attributeWatcherFailed,
@@ -253,6 +247,7 @@ describe('watchAttributes', () => {
       },
     ]);
     expect(changes).toEqual([{ name: VIRTUAL_ATTRIBUTE, value: 'open()', previousValue: null }]);
+    log.stop();
   });
 
   it('delivers after declared options in the same mutation batch', async () => {
@@ -317,7 +312,7 @@ describe('watchAttributes', () => {
     await whenDOMSettled();
 
     expect(instance.$isMounted).toBe(false);
-    expect(el[INSTANCES]?.has(name)).toBe(false);
+    expect(getInstance(el, name)).toBeUndefined();
     expect(calls).toEqual(['cleanup']);
   });
 
@@ -465,7 +460,7 @@ describe('setDOMMutationProcessor', () => {
 
     // The registry installed the one processor at import time; a second caller
     // does not replace it and does not get a second delivery of the batch.
-    expect(el[INSTANCES]?.get(name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
     expect(later).not.toHaveBeenCalled();
   });
 });
diff --git a/packages/v4/src/group.spec.ts b/packages/v4/src/group.spec.ts
index 467859e33..f0e2ca91a 100644
--- a/packages/v4/src/group.spec.ts
+++ b/packages/v4/src/group.spec.ts
@@ -211,11 +211,11 @@ function disclosureMarkup(id: string, open = false): string {
 }
 
 function disclosure(root: ParentNode, id: string): Disclosure {
-  return getInstance<Disclosure>(root.querySelector(`#${id}`)!, 'Disclosure')!;
+  return getInstance<Disclosure>(root.querySelector(`#${id}`), 'Disclosure')!;
 }
 
 function group(root: ParentNode, id: string): DisclosureGroup {
-  return getInstance<DisclosureGroup>(root.querySelector(`#${id}`)!, 'DisclosureGroup')!;
+  return getInstance<DisclosureGroup>(root.querySelector(`#${id}`), 'DisclosureGroup')!;
 }
 
 describe('a group of disclosures', () => {
diff --git a/packages/v4/src/instances.spec.ts b/packages/v4/src/instances.spec.ts
index 363541a9f..46ef1c48a 100644
--- a/packages/v4/src/instances.spec.ts
+++ b/packages/v4/src/instances.spec.ts
@@ -87,6 +87,9 @@ describe('getInstances by name', () => {
     await settle();
 
     expect(el.matches('[data-component~="Unregistered"]')).toBe(true);
+    // The raw read is the premise, not the assertion: it states that no map
+    // exists at all, so the empty result below cannot be an empty map. A
+    // lookup cannot say that, and would test these exports with themselves.
     expect(el[INSTANCES]).toBeUndefined();
     expect(getInstances('Unregistered')).toEqual([]);
   });
diff --git a/packages/v4/src/instances.ts b/packages/v4/src/instances.ts
index a7f472fb2..96b4d1d0f 100644
--- a/packages/v4/src/instances.ts
+++ b/packages/v4/src/instances.ts
@@ -120,10 +120,25 @@ export function getUnmountedInstances<T extends Base = Base>(
  * answer to "is this element's instance there yet", which every plural form
  * loses by returning a list.
  *
+ * `el` accepts `null` so that a `querySelector()` result can be passed
+ * straight through. The two ways of having no instance — no element, and an
+ * element without one — are the same answer to the caller, and `undefined`
+ * says it for both. Narrowing the parameter to `Element` would buy nothing
+ * back: the body reads an optional map either way, so the only thing a
+ * stricter type produces is a `!` at every call site that asserts something
+ * the function never needed.
+ *
+ * The *return* stays `T | undefined`, and that asymmetry is deliberate. An
+ * absent element is a fact the caller may reasonably not know; an absent
+ * instance is a fact the caller must handle.
+ *
  * There is deliberately no `getMountedInstance`. The result is one object, so
  * a caller who needs the live one reads `.$isMounted` on it — a second export
  * would only hide that check behind a `undefined` that means two things.
  */
-export function getInstance<T extends Base = Base>(el: Element, name: string): T | undefined {
-  return el[INSTANCES]?.get(name) as T | undefined;
+export function getInstance<T extends Base = Base>(
+  el: Element | null,
+  name: string,
+): T | undefined {
+  return el?.[INSTANCES]?.get(name) as T | undefined;
 }
diff --git a/packages/v4/src/mount-strategies.spec.ts b/packages/v4/src/mount-strategies.spec.ts
index e49cb2d2b..c5dbbe080 100644
--- a/packages/v4/src/mount-strategies.spec.ts
+++ b/packages/v4/src/mount-strategies.spec.ts
@@ -2,10 +2,10 @@ import { afterEach, describe, expect, it, vi } from 'vitest';
 import { Base, type BaseConfig } from './Base.js';
 import { DIAGNOSTICS, type ToolkitDiagnosticDetail } from './diagnostic-contract.js';
 import { EVENTS } from './events.js';
-import { INSTANCES } from './protocol-symbols.js';
+import { getInstance } from './instances.js';
 import { registerComponent } from './registry.js';
 import { getSharedRuntimeSlot } from './shared-runtime.js';
-import { resetDom, settle, waitFor } from './test/index.js';
+import { captureDiagnostics, resetDom, settle, waitFor } from './test/index.js';
 
 /**
  * The page-wide interaction signal is a fact about the visit, so it survives a
@@ -67,10 +67,6 @@ function render(name: string, attributes: Record<string, string> = {}, style = O
   return el;
 }
 
-function instanceOf<T extends Base>(el: Element, name: string): T | undefined {
-  return el[INSTANCES]?.get(name) as T | undefined;
-}
-
 /**
  * A bounded quiet period, for the assertions that nothing mounted. Every
  * positive wait below polls for the state it expects; an absence cannot be
@@ -93,7 +89,7 @@ describe('eager (default)', () => {
     const el = render(name);
     await settle();
 
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 });
 
@@ -103,7 +99,7 @@ describe('data-mount="visible"', () => {
     const el = render(name, { 'data-mount': 'visible' }, OFFSCREEN);
     await quiet();
 
-    expect(el[INSTANCES]?.get(name)).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
   });
 
   it('mounts once with a root margin and stays mounted afterwards', async () => {
@@ -112,7 +108,7 @@ describe('data-mount="visible"', () => {
     await quiet();
 
     el.setAttribute('style', ONSCREEN);
-    const instance = await waitFor(() => instanceOf(el, name));
+    const instance = await waitFor(() => getInstance(el, name));
     expect(instance?.$isMounted).toBe(true);
 
     el.setAttribute('style', OFFSCREEN);
@@ -126,7 +122,7 @@ describe('data-mount="in-view"', () => {
     const { name, Tracked } = defineTracked();
     type Tracked = InstanceType<typeof Tracked>;
     const el = render(name, { 'data-mount': 'in-view' }, ONSCREEN);
-    const instance = await waitFor(() => instanceOf<Tracked>(el, name));
+    const instance = await waitFor(() => getInstance<Tracked>(el, name));
 
     expect(instance?.$isMounted).toBe(true);
     expect(instance?.mounts).toBe(1);
@@ -138,7 +134,7 @@ describe('data-mount="in-view"', () => {
 
     el.setAttribute('style', ONSCREEN);
     await waitFor(() => instance?.mounts === 2);
-    expect(instanceOf(el, name)).toBe(instance);
+    expect(getInstance(el, name)).toBe(instance);
     expect(instance?.$isMounted).toBe(true);
     expect(instance?.mounts).toBe(2);
   });
@@ -152,11 +148,11 @@ describe('data-mount="interaction"', () => {
     // cursor — which the top-left corner of the viewport often is.
     const el = render(name, { 'data-mount': 'interaction' }, OFFSCREEN);
     await settle();
-    expect(el[INSTANCES]?.get(name)).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
 
     el.dispatchEvent(new PointerEvent('pointerdown', { bubbles: true }));
     await settle();
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 });
 
@@ -167,16 +163,16 @@ describe('data-mount="interaction:page"', () => {
     const { name } = defineTracked();
     const el = render(name, { 'data-mount': 'interaction:page' }, OFFSCREEN);
     await settle();
-    expect(el[INSTANCES]?.get(name)).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
 
     // What the element-scoped strategy mounts on says nothing about the page.
     el.dispatchEvent(new PointerEvent('pointerenter'));
     await settle();
-    expect(el[INSTANCES]?.get(name)).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
 
     document.body.dispatchEvent(new PointerEvent('pointerdown', { bubbles: true }));
     await settle();
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 
   it('mounts every waiting element from one interaction, wherever it lands', async () => {
@@ -188,8 +184,8 @@ describe('data-mount="interaction:page"', () => {
     document.dispatchEvent(new KeyboardEvent('keydown', { key: 'Tab', bubbles: true }));
     await settle();
 
-    expect(instanceOf(first, name)?.$isMounted).toBe(true);
-    expect(instanceOf(second, name)?.$isMounted).toBe(true);
+    expect(getInstance(first, name)?.$isMounted).toBe(true);
+    expect(getInstance(second, name)?.$isMounted).toBe(true);
   });
 
   it('listens once for the page rather than once per waiting element', async () => {
@@ -236,7 +232,7 @@ describe('data-mount="interaction:page"', () => {
     const late = render(name, { 'data-mount': 'interaction:page' }, OFFSCREEN);
     await settle();
 
-    expect(instanceOf(late, name)?.$isMounted).toBe(true);
+    expect(getInstance(late, name)?.$isMounted).toBe(true);
   });
 
   it('stops waiting for an element that left before the interaction', async () => {
@@ -249,7 +245,7 @@ describe('data-mount="interaction:page"', () => {
     document.body.dispatchEvent(new PointerEvent('pointerdown', { bubbles: true }));
     await settle();
 
-    expect(el[INSTANCES]?.get(name)).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
   });
 });
 
@@ -257,9 +253,9 @@ describe('data-mount="idle"', () => {
   it('mounts when the main thread goes idle', async () => {
     const { name } = defineTracked();
     const el = render(name, { 'data-mount': 'idle' });
-    await waitFor(() => instanceOf(el, name)?.$isMounted);
+    await waitFor(() => getInstance(el, name)?.$isMounted);
 
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 });
 
@@ -273,15 +269,15 @@ describe('data-mount="media:…"', () => {
     // makes the failing query's absence below mean something.
     await waitFor(
       () =>
-        instanceOf(document.querySelector(`[data-component="${matching.name}"]`)!, matching.name)
+        getInstance(document.querySelector(`[data-component="${matching.name}"]`), matching.name)
           ?.$isMounted,
     );
 
     expect(
-      instanceOf(document.querySelector(`[data-component="${matching.name}"]`)!, matching.name)
+      getInstance(document.querySelector(`[data-component="${matching.name}"]`), matching.name)
         ?.$isMounted,
     ).toBe(true);
-    expect(narrow[INSTANCES]?.get(failing.name)).toBeUndefined();
+    expect(getInstance(narrow, failing.name)).toBeUndefined();
   });
 });
 
@@ -290,11 +286,11 @@ describe('config.mountStrategy', () => {
     const { name } = defineTracked({ mountStrategy: 'visible:200px 0px' });
     const el = render(name, {}, OFFSCREEN);
     await quiet();
-    expect(el[INSTANCES]?.get(name)).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
 
     el.setAttribute('style', ONSCREEN);
-    await waitFor(() => instanceOf(el, name)?.$isMounted);
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    await waitFor(() => getInstance(el, name)?.$isMounted);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 
   it('is overridden by the element attribute', async () => {
@@ -302,17 +298,17 @@ describe('config.mountStrategy', () => {
     const el = render(name, { 'data-mount': 'eager' }, OFFSCREEN);
     await settle();
 
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 
   it('lets a parameterized data-mount override the component config', async () => {
     const { name } = defineTracked({ mountStrategy: 'eager' });
     const el = render(name, { 'data-mount': 'in-view:200px 0px' }, OFFSCREEN);
     await quiet();
-    expect(el[INSTANCES]?.get(name)).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
 
     el.setAttribute('style', ONSCREEN);
-    const instance = await waitFor(() => instanceOf(el, name));
+    const instance = await waitFor(() => getInstance(el, name));
     expect(instance?.$isMounted).toBe(true);
 
     el.setAttribute('style', OFFSCREEN);
@@ -324,12 +320,12 @@ describe('config.mountStrategy', () => {
     const { name } = defineTracked({ mountStrategy: 'visible' });
     const el = render(name, { 'data-mount': 'interaction' }, OFFSCREEN);
     await settle();
-    expect(el[INSTANCES]?.get(name)).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
 
     el.removeAttribute('data-mount');
     el.setAttribute('style', ONSCREEN);
-    await waitFor(() => instanceOf(el, name)?.$isMounted);
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    await waitFor(() => getInstance(el, name)?.$isMounted);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 
   it('is inherited by a subclass that declares a config of its own', async () => {
@@ -344,11 +340,11 @@ describe('config.mountStrategy', () => {
 
     const el = render(name, {}, OFFSCREEN);
     await quiet();
-    expect(el[INSTANCES]?.get(name)).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
 
     el.setAttribute('style', ONSCREEN);
-    await waitFor(() => instanceOf(el, name)?.$isMounted);
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    await waitFor(() => getInstance(el, name)?.$isMounted);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 
   it('is overridden by a subclass declaring its own strategy', async () => {
@@ -363,7 +359,7 @@ describe('config.mountStrategy', () => {
 
     const el = render(name, {}, OFFSCREEN);
     await settle();
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 });
 
@@ -372,11 +368,11 @@ describe('dynamic data-mount', () => {
     const { name } = defineTracked();
     const el = render(name, { 'data-mount': 'visible' }, OFFSCREEN);
     await quiet();
-    expect(el[INSTANCES]?.get(name)).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
 
     el.setAttribute('data-mount', 'eager');
     await settle();
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 });
 
@@ -397,7 +393,7 @@ describe('invalid data-mount', () => {
 
       await waitFor(() => events.length > 0);
 
-      expect(el[INSTANCES]?.get(name)).toBeUndefined();
+      expect(getInstance(el, name)).toBeUndefined();
       expect(events).toHaveLength(1);
       const failure = events[0].detail.error;
       expect(failure).toBeInstanceOf(Error);
@@ -422,31 +418,28 @@ describe('invalid data-mount', () => {
 
       el.setAttribute('data-mount', 'eager');
       await settle();
-      expect(instanceOf(el, name)?.$isMounted).toBe(true);
+      expect(getInstance(el, name)?.$isMounted).toBe(true);
     },
   );
 
   it('does not stop a valid sibling in the same reconciliation batch', async () => {
     const broken = defineTracked();
     const healthy = defineTracked();
-    const diagnostics: ToolkitDiagnosticDetail[] = [];
 
     const brokenEl = document.createElement('div');
     brokenEl.setAttribute('data-component', broken.name);
     brokenEl.setAttribute('data-mount', 'in-view:not-a-root-margin');
-    brokenEl.addEventListener(EVENTS.diagnostic, (event) => {
-      event.preventDefault();
-      diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-    });
+    const log = captureDiagnostics(brokenEl);
     const healthyEl = document.createElement('div');
     healthyEl.setAttribute('data-component', healthy.name);
     document.body.append(brokenEl, healthyEl);
 
-    await waitFor(() => instanceOf(healthyEl, healthy.name)?.$isMounted);
+    await waitFor(() => getInstance(healthyEl, healthy.name)?.$isMounted);
 
-    expect(brokenEl[INSTANCES]?.get(broken.name)).toBeUndefined();
-    expect(diagnostics).toHaveLength(1);
-    expect(instanceOf(healthyEl, healthy.name)?.$isMounted).toBe(true);
+    expect(getInstance(brokenEl, broken.name)).toBeUndefined();
+    expect(log.entries).toHaveLength(1);
+    expect(getInstance(healthyEl, healthy.name)?.$isMounted).toBe(true);
+    log.stop();
   });
 });
 
@@ -460,7 +453,7 @@ describe('teardown', () => {
     await quiet();
     el.setAttribute('style', ONSCREEN);
     await quiet();
-    expect(el[INSTANCES]?.get(name)).toBeUndefined();
+    expect(getInstance(el, name)).toBeUndefined();
   });
 
   it('re-schedules an element moved in a single batch', async () => {
@@ -472,7 +465,7 @@ describe('teardown', () => {
     const el = document.createElement('div');
     el.setAttribute('data-component', name);
     from.append(el);
-    const instance = await waitFor(() => instanceOf<Tracked>(el, name));
+    const instance = await waitFor(() => getInstance<Tracked>(el, name));
     expect(instance?.$isMounted).toBe(true);
     expect(instance?.mounts).toBe(1);
 
@@ -480,7 +473,7 @@ describe('teardown', () => {
     // mount cycle and starts another without replacing the instance.
     to.append(el);
     await waitFor(() => instance?.mounts === 2);
-    expect(instanceOf(el, name)).toBe(instance);
+    expect(getInstance(el, name)).toBe(instance);
     expect(instance?.$isMounted).toBe(true);
     expect(instance?.unmounts).toBe(1);
     expect(instance?.mounts).toBe(2);
@@ -490,14 +483,14 @@ describe('teardown', () => {
   it('re-schedules an element that comes back', async () => {
     const { name } = defineTracked();
     const el = render(name, { 'data-mount': 'visible' }, ONSCREEN);
-    await waitFor(() => instanceOf(el, name)?.$isMounted);
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    await waitFor(() => getInstance(el, name)?.$isMounted);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
 
     el.remove();
-    await waitFor(() => instanceOf(el, name)?.$isMounted === false);
+    await waitFor(() => getInstance(el, name)?.$isMounted === false);
     document.body.append(el);
-    await waitFor(() => instanceOf(el, name)?.$isMounted);
-    expect(instanceOf(el, name)?.$isMounted).toBe(true);
+    await waitFor(() => getInstance(el, name)?.$isMounted);
+    expect(getInstance(el, name)?.$isMounted).toBe(true);
   });
 });
 
@@ -513,12 +506,12 @@ describe('several components on one element', () => {
     document.body.append(el);
     await quiet();
 
-    expect(el[INSTANCES]?.get(first.name)).toBeUndefined();
-    expect(el[INSTANCES]?.get(second.name)).toBeUndefined();
+    expect(getInstance(el, first.name)).toBeUndefined();
+    expect(getInstance(el, second.name)).toBeUndefined();
 
     el.setAttribute('style', ONSCREEN);
-    await waitFor(() => instanceOf(el, first.name)?.$isMounted);
-    expect(instanceOf(el, first.name)?.$isMounted).toBe(true);
-    expect(instanceOf(el, second.name)?.$isMounted).toBe(true);
+    await waitFor(() => getInstance(el, first.name)?.$isMounted);
+    expect(getInstance(el, first.name)?.$isMounted).toBe(true);
+    expect(getInstance(el, second.name)?.$isMounted).toBe(true);
   });
 });
diff --git a/packages/v4/src/negotiated-events.spec.ts b/packages/v4/src/negotiated-events.spec.ts
index e9fae220f..845777af1 100644
--- a/packages/v4/src/negotiated-events.spec.ts
+++ b/packages/v4/src/negotiated-events.spec.ts
@@ -18,19 +18,15 @@ afterEach(async () => {
   await resetDom();
 });
 
+/** {@link captureDiagnostics} bounded to one awaited call. */
 async function catchDiagnostics(run: () => Promise<void>): Promise<ToolkitDiagnosticDetail[]> {
-  const diagnostics: ToolkitDiagnosticDetail[] = [];
-  const onDiagnostic = (event: Event) => {
-    event.preventDefault();
-    diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-  };
-  document.addEventListener(EVENTS.diagnostic, onDiagnostic);
+  const log = captureDiagnostics();
   try {
     await run();
   } finally {
-    document.removeEventListener(EVENTS.diagnostic, onDiagnostic);
+    log.stop();
   }
-  return diagnostics;
+  return log.entries;
 }
 
 function renderTarget() {
@@ -163,11 +159,7 @@ describe('domUpdate()', () => {
     // event suppresses it — so this one keeps its spy. `captureDiagnostics()`
     // cancels every event it sees, which would make the assertion vacuous.
     const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
-    const diagnostics: ToolkitDiagnosticDetail[] = [];
-    target.addEventListener(EVENTS.diagnostic, (event) => {
-      event.preventDefault();
-      diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-    });
+    const log = captureDiagnostics(target);
     const runner = () => Promise.resolve();
     claim(outer, runner);
 
@@ -175,8 +167,9 @@ describe('domUpdate()', () => {
     await domUpdate(target, () => target.setAttribute('data-applied-again', 'yes'));
 
     expect(target.dataset).toMatchObject({ applied: 'yes', appliedAgain: 'yes' });
-    expect(diagnostics.map(({ code }) => code)).toEqual([DIAGNOSTICS.protocol.unappliedDomUpdate]);
+    expect(log.codes).toEqual([DIAGNOSTICS.protocol.unappliedDomUpdate]);
     expect(warn).not.toHaveBeenCalled();
+    log.stop();
   });
 
   it('warns and ignores a wrap() registration made after dispatch', async () => {
diff --git a/packages/v4/src/props.spec.ts b/packages/v4/src/props.spec.ts
index 7d2a48ef2..ffcc486cd 100644
--- a/packages/v4/src/props.spec.ts
+++ b/packages/v4/src/props.spec.ts
@@ -174,7 +174,7 @@ describe('a component declared with a props type parameter', () => {
       </form>`;
     await settle();
 
-    const instance = getInstance<Extensible>(document.querySelector('form')!, 'Extensible')!;
+    const instance = getInstance<Extensible>(document.querySelector('form'), 'Extensible')!;
     expect(instance.$options.target).toBe('here');
     expect(instance.$refs.btn).toBeInstanceOf(HTMLButtonElement);
     expect(instance.$refs.items).toHaveLength(2);
diff --git a/packages/v4/src/registry.spec.ts b/packages/v4/src/registry.spec.ts
index 6b1f33072..349fb78f5 100644
--- a/packages/v4/src/registry.spec.ts
+++ b/packages/v4/src/registry.spec.ts
@@ -3,7 +3,6 @@ import { Base, type BaseConfig } from './Base.js';
 import { DIAGNOSTICS, type ToolkitDiagnosticDetail } from './diagnostic-contract.js';
 import { EVENTS } from './events.js';
 import { getInstance } from './instances.js';
-import { INSTANCES } from './protocol-symbols.js';
 import { registerComponent } from './registry.js';
 import { renderTodoList, TodoItem, TodoList } from './todo.fixtures.js';
 import { captureDiagnostics, resetDom, settle } from './test/index.js';
@@ -54,7 +53,7 @@ describe('registry', () => {
     const el = document.createElement('li');
     document.body.append(el);
     await settle();
-    expect(el[INSTANCES]?.get('TodoItem')).toBeUndefined();
+    expect(getInstance(el, 'TodoItem')).toBeUndefined();
 
     el.setAttribute('data-component', 'TodoItem');
     await settle();
@@ -75,8 +74,8 @@ describe('registry', () => {
     el.setAttribute('data-component', 'TodoCount');
     await settle();
 
-    expect(el[INSTANCES]?.get('TodoItem')).toBeUndefined();
-    expect(el[INSTANCES]?.get('TodoCount')).toBe(count);
+    expect(getInstance(el, 'TodoItem')).toBeUndefined();
+    expect(getInstance(el, 'TodoCount')).toBe(count);
     expect(count.$isMounted).toBe(true);
   });
 
@@ -89,7 +88,7 @@ describe('registry', () => {
 
     el.removeAttribute('data-component');
     await settle();
-    expect(el[INSTANCES]?.get('TodoItem')).toBeUndefined();
+    expect(getInstance(el, 'TodoItem')).toBeUndefined();
     expect(first.$isMounted).toBe(false);
 
     el.setAttribute('data-component', 'TodoItem');
@@ -142,7 +141,7 @@ describe('registry', () => {
     await settle();
 
     expect(attempts).toHaveLength(1);
-    expect(broken[INSTANCES]?.get('BrokenConstructionErrorEvent')).toBeUndefined();
+    expect(getInstance(broken, 'BrokenConstructionErrorEvent')).toBeUndefined();
     expect(brokenMounts).toBe(0);
     expect(healthyMounts).toBe(1);
 
@@ -154,7 +153,7 @@ describe('registry', () => {
 
     expect(attempts).toHaveLength(2);
     expect(attempts[1]).not.toBe(firstAttempt);
-    expect(broken[INSTANCES]?.get('BrokenConstructionErrorEvent')).toBeUndefined();
+    expect(getInstance(broken, 'BrokenConstructionErrorEvent')).toBeUndefined();
     expect(brokenMounts).toBe(0);
     expect(events).toHaveLength(2);
     expect(events.every((event) => event.target === broken)).toBe(true);
@@ -170,7 +169,6 @@ describe('registry', () => {
 
   it('preserves a valid pre-existing instance when its registry mount fails', async () => {
     const failure = new Error('mount failed');
-    const diagnostics: ToolkitDiagnosticDetail[] = [];
 
     class Existing extends Base {
       static config = { name: 'ExistingMountFailure' };
@@ -183,22 +181,20 @@ describe('registry', () => {
     registerComponent(Existing);
     const el = document.createElement('div');
     const instance = new Existing(el);
-    el.addEventListener(EVENTS.diagnostic, (event) => {
-      event.preventDefault();
-      diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-    });
+    const log = captureDiagnostics(el);
     el.setAttribute('data-component', 'ExistingMountFailure');
     document.body.append(el);
     await settle();
 
-    expect(el[INSTANCES]?.get('ExistingMountFailure')).toBe(instance);
-    expect(diagnostics).toHaveLength(1);
-    expect(diagnostics[0]).toMatchObject({
+    expect(getInstance(el, 'ExistingMountFailure')).toBe(instance);
+    expect(log.entries).toHaveLength(1);
+    expect(log.entries[0]).toMatchObject({
       severity: 'error',
       code: DIAGNOSTICS.component.mountFailed,
       error: failure,
       component: 'ExistingMountFailure',
     });
+    log.stop();
   });
 
   it('processes a pending token replacement before mounting a newly registered class', async () => {
@@ -213,9 +209,7 @@ describe('registry', () => {
     class After extends Base {
       static config = { name: 'RegistrationAfter' };
       mounted(): void {
-        calls.push(
-          `after:mounted:before=${Boolean(this.$el[INSTANCES]?.has('RegistrationBefore'))}`,
-        );
+        calls.push(`after:mounted:before=${Boolean(getInstance(this.$el, 'RegistrationBefore'))}`);
       }
     }
 
diff --git a/packages/v4/src/responsive-components.spec.ts b/packages/v4/src/responsive-components.spec.ts
index f99cd1ba0..ff7a0fe54 100644
--- a/packages/v4/src/responsive-components.spec.ts
+++ b/packages/v4/src/responsive-components.spec.ts
@@ -2,8 +2,7 @@ import { afterEach, describe, expect, it } from 'vitest';
 import { Base, type BaseConfig } from './Base.js';
 import { DIAGNOSTICS } from './diagnostic-contract.js';
 import { whenDOMSettled } from './dom-mutations.js';
-import { getMountedInstances } from './instances.js';
-import { INSTANCES } from './protocol-symbols.js';
+import { getInstance, getInstances, getMountedInstances } from './instances.js';
 import { registerComponent, registerManifest } from './registry.js';
 import { BREAKPOINTS, setBreakpoints } from './services/breakpoint.js';
 import { captureDiagnostics, resetDom, settle } from './test/index.js';
@@ -53,7 +52,7 @@ function render(attributes: Record<string, string>): HTMLElement {
 }
 
 function instance(el: Element, name: string): TrackedComponent | undefined {
-  return el[INSTANCES]?.get(name) as TrackedComponent | undefined;
+  return getInstance<TrackedComponent>(el, name);
 }
 
 function at(name: 'small' | 'middle' | 'wide'): void {
@@ -105,9 +104,11 @@ describe('responsive component declarations', () => {
     });
     await whenDOMSettled();
 
-    expect([...(el[INSTANCES]?.keys() ?? [])].sort()).toEqual(
-      [action.name, analytics.name, mobileMenu.name, mobileSearch.name].sort(),
-    );
+    expect(
+      getInstances(el)
+        .map((found) => found.$config.name)
+        .sort(),
+    ).toEqual([action.name, analytics.name, mobileMenu.name, mobileSearch.name].sort());
     expect(instance(el, action.name)?.mounts).toBe(1);
     expect(getMountedInstances(mobileMenu.name)).toEqual([instance(el, mobileMenu.name)]);
   });
diff --git a/packages/v4/src/responsive-options.spec.ts b/packages/v4/src/responsive-options.spec.ts
index 9582e87c9..4caee4a42 100644
--- a/packages/v4/src/responsive-options.spec.ts
+++ b/packages/v4/src/responsive-options.spec.ts
@@ -138,7 +138,7 @@ describe('responsive options', () => {
           data-option-label:large="wide"></p>`,
     );
     await settle();
-    const label = getInstance<Label>(root.firstElementChild!, 'Label')!;
+    const label = getInstance<Label>(root.firstElementChild, 'Label')!;
 
     expect(label.$options.label).toBe('base');
 
@@ -151,7 +151,7 @@ describe('responsive options', () => {
     const root = render('<p data-component="Label"></p>');
     await settle();
 
-    expect(getInstance<Label>(root.firstElementChild!, 'Label')!.$options.label).toBe('base');
+    expect(getInstance<Label>(root.firstElementChild, 'Label')!.$options.label).toBe('base');
   });
 
   it('is derived on read, so a component that never subscribes still sees the crossing', async () => {
@@ -162,7 +162,7 @@ describe('responsive options', () => {
           data-option-label:large="wide"></p>`,
     );
     await settle();
-    const label = getInstance<Label>(root.firstElementChild!, 'Label')!;
+    const label = getInstance<Label>(root.firstElementChild, 'Label')!;
     expect(label.$options.label).toBe('narrow');
 
     const added = await countMediaListeners(async () => {
@@ -181,7 +181,7 @@ describe('responsive options', () => {
           data-option-label:large="wide"></p>`,
     );
     await settle();
-    const label = getInstance<Label>(root.firstElementChild!, 'Label')!;
+    const label = getInstance<Label>(root.firstElementChild, 'Label')!;
 
     const added = await countMediaListeners(async () => {
       for (let index = 0; index < 5; index += 1) {
@@ -208,7 +208,7 @@ describe('responsive options', () => {
             data-option-columns:large="4"></div>`,
     );
     await settle();
-    const grid = getInstance<Grid>(root.firstElementChild!, 'Grid')!;
+    const grid = getInstance<Grid>(root.firstElementChild, 'Grid')!;
 
     expect(grid.changes).toHaveLength(1);
     expect(grid.changes[0]).toMatchObject({ value: 1, initial: true });
@@ -232,7 +232,7 @@ describe('responsive options', () => {
     atSmall();
     const root = render('<div data-component="Grid" data-option-columns="2"></div>');
     await settle();
-    const grid = getInstance<Grid>(root.firstElementChild!, 'Grid')!;
+    const grid = getInstance<Grid>(root.firstElementChild, 'Grid')!;
     expect(grid.changes).toHaveLength(1);
 
     atLarge();
@@ -250,7 +250,7 @@ describe('responsive options', () => {
             data-option-columns:large="4"></div>`,
     );
     await settle();
-    const grid = getInstance<Grid>(root.firstElementChild!, 'Grid')!;
+    const grid = getInstance<Grid>(root.firstElementChild, 'Grid')!;
     grid.changes = [];
 
     grid.$el.setAttribute('data-option-columns:large', '6');
@@ -274,7 +274,7 @@ describe('responsive options', () => {
       `<div data-component="Grid" data-option-columns:large="4" data-option-gap="8"></div>`,
     );
     await settle();
-    const grid = getInstance<Grid>(root.firstElementChild!, 'Grid')!;
+    const grid = getInstance<Grid>(root.firstElementChild, 'Grid')!;
 
     expect(grid.$options.gap).toBe(8);
     atLarge();
@@ -290,7 +290,7 @@ describe('responsive options', () => {
           data-option-theme:large="dark"></p>`,
     );
     await settle();
-    const banner = getInstance<Banner>(root.firstElementChild!, 'Banner')!;
+    const banner = getInstance<Banner>(root.firstElementChild, 'Banner')!;
 
     expect(banner.$options.theme).toBe('light');
     atLarge();
@@ -301,7 +301,7 @@ describe('responsive options', () => {
     atSmall();
     const root = render('<p data-component="Banner" data-option-theme="light"></p>');
     await settle();
-    const banner = getInstance<Banner>(root.firstElementChild!, 'Banner')!;
+    const banner = getInstance<Banner>(root.firstElementChild, 'Banner')!;
     banner.changes = [];
 
     banner.$el.setAttribute('data-option-theme:small', 'dark');
@@ -315,7 +315,7 @@ describe('responsive options', () => {
     atSmall();
     const root = render('<div data-component="Grid" data-option-columns="1"></div>');
     await settle();
-    const grid = getInstance<Grid>(root.firstElementChild!, 'Grid')!;
+    const grid = getInstance<Grid>(root.firstElementChild, 'Grid')!;
     grid.changes = [];
 
     setBreakpoints({ small: '0rem', wide: '0rem' });
@@ -341,7 +341,7 @@ describe('responsive options', () => {
     });
     expect(added).toBeGreaterThan(0);
 
-    const grid = getInstance<Grid>(root.firstElementChild!, 'Grid')!;
+    const grid = getInstance<Grid>(root.firstElementChild, 'Grid')!;
     atLarge();
     expect(grid.changes).toHaveLength(2);
 
@@ -385,7 +385,7 @@ describe('responsive options', () => {
             data-option-columns:large="4"></div>`,
     );
     await settle();
-    const grid = getInstance<LazyGrid>(root.firstElementChild!, 'LazyGrid')!;
+    const grid = getInstance<LazyGrid>(root.firstElementChild, 'LazyGrid')!;
 
     expect(grid.$options.columns).toBe(1);
     grid.changes = [];
@@ -469,7 +469,7 @@ describe('negated boolean options', () => {
   it('turns a boolean off by the presence of its negated attribute', async () => {
     const root = render(`<p data-component="Flags" data-option-no-view-transition></p>`);
     await settle();
-    const flags = getInstance<Flags>(root.firstElementChild!, 'Flags')!;
+    const flags = getInstance<Flags>(root.firstElementChild, 'Flags')!;
 
     expect(flags.$options.viewTransition).toBe(false);
   });
@@ -506,7 +506,7 @@ describe('negated boolean options', () => {
     await settle();
 
     // A negation is a flag too: it is present or it is not.
-    expect(getInstance<Flags>(root.firstElementChild!, 'Flags')!.$options.viewTransition).toBe(
+    expect(getInstance<Flags>(root.firstElementChild, 'Flags')!.$options.viewTransition).toBe(
       false,
     );
   });
@@ -515,7 +515,7 @@ describe('negated boolean options', () => {
     const root = render(`<p data-component="Flags" data-option-no-mode></p>`);
     await settle();
 
-    expect(getInstance<Flags>(root.firstElementChild!, 'Flags')!.$options.mode).toBe(false);
+    expect(getInstance<Flags>(root.firstElementChild, 'Flags')!.$options.mode).toBe(false);
   });
 
   it('turns one off whatever order the union declares its members in', async () => {
@@ -523,14 +523,14 @@ describe('negated boolean options', () => {
     await settle();
 
     // `[String, Boolean]` would otherwise read the negation as a string.
-    expect(getInstance<Flags>(root.firstElementChild!, 'Flags')!.$options.label).toBe(false);
+    expect(getInstance<Flags>(root.firstElementChild, 'Flags')!.$options.label).toBe(false);
   });
 
   it('cascades from a breakpoint like every other spelling', async () => {
     atSmall();
     const root = render(`<p data-component="Flags" data-option-no-view-transition:large></p>`);
     await settle();
-    const flags = getInstance<Flags>(root.firstElementChild!, 'Flags')!;
+    const flags = getInstance<Flags>(root.firstElementChild, 'Flags')!;
 
     expect(flags.$options.viewTransition).toBe(true);
 
@@ -546,7 +546,7 @@ describe('negated boolean options', () => {
           data-option-view-transition:large="true"></p>`,
     );
     await settle();
-    const flags = getInstance<Flags>(root.firstElementChild!, 'Flags')!;
+    const flags = getInstance<Flags>(root.firstElementChild, 'Flags')!;
 
     expect(flags.$options.viewTransition).toBe(false);
 
@@ -563,15 +563,13 @@ describe('negated boolean options', () => {
     await settle();
 
     // An explicit value states more than a flag does.
-    expect(getInstance<Flags>(root.firstElementChild!, 'Flags')!.$options.viewTransition).toBe(
-      true,
-    );
+    expect(getInstance<Flags>(root.firstElementChild, 'Flags')!.$options.viewTransition).toBe(true);
   });
 
   it('announces a change to the negated attribute like any other option change', async () => {
     const root = render(`<p data-component="Flags"></p>`);
     await settle();
-    const flags = getInstance<Flags>(root.firstElementChild!, 'Flags')!;
+    const flags = getInstance<Flags>(root.firstElementChild, 'Flags')!;
     expect(flags.$options.viewTransition).toBe(true);
 
     (root.firstElementChild as HTMLElement).setAttribute('data-option-no-view-transition', '');
@@ -599,6 +597,6 @@ describe('negated boolean options', () => {
     await settle();
 
     // Nothing to turn off, so the attribute is not this option's: the default stands.
-    expect(getInstance<Label>(root.firstElementChild!, 'Label')!.$options.label).toBe('base');
+    expect(getInstance<Label>(root.firstElementChild, 'Label')!.$options.label).toBe('base');
   });
 });
diff --git a/packages/v4/src/scheduler.spec.ts b/packages/v4/src/scheduler.spec.ts
index f0a13d849..bb6047543 100644
--- a/packages/v4/src/scheduler.spec.ts
+++ b/packages/v4/src/scheduler.spec.ts
@@ -1,7 +1,7 @@
 import { describe, expect, it, vi } from 'vitest';
-import { DIAGNOSTICS, type ToolkitDiagnosticDetail } from './diagnostic-contract.js';
-import { EVENTS } from './events.js';
+import { DIAGNOSTICS } from './diagnostic-contract.js';
 import { nextFrame, defaultScheduler, type SchedulerPhase, type TickProps } from './scheduler.js';
+import { captureDiagnostics } from './test/index.js';
 
 describe('defaultScheduler (real frames)', () => {
   it('runs reads before writes within one frame, regardless of scheduling order', async () => {
@@ -26,15 +26,7 @@ describe('defaultScheduler (real frames)', () => {
 
   it('survives a throwing task, reports it, and keeps its promise rejection', async () => {
     const failure = new Error('boom');
-    const diagnostics: ToolkitDiagnosticDetail[] = [];
-    document.addEventListener(
-      EVENTS.diagnostic,
-      (event) => {
-        event.preventDefault();
-        diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-      },
-      { once: true },
-    );
+    const log = captureDiagnostics();
 
     const task = defaultScheduler.write(() => {
       throw failure;
@@ -43,7 +35,7 @@ describe('defaultScheduler (real frames)', () => {
 
     const after = defaultScheduler.write(() => 'still alive');
     await expect(after.promise).resolves.toBe('still alive');
-    expect(diagnostics).toEqual([
+    expect(log.entries).toEqual([
       {
         severity: 'error',
         code: DIAGNOSTICS.callback.scheduledTaskFailed,
@@ -51,6 +43,7 @@ describe('defaultScheduler (real frames)', () => {
         error: failure,
       },
     ]);
+    log.stop();
   });
 
   it('resolves task promises with return values and supports cancel', async () => {
@@ -159,15 +152,7 @@ describe('defaultScheduler.background', () => {
 
   it('reports a native background-post rejection and continues through the fallback', async () => {
     const failure = new Error('native post failed');
-    const diagnostics: ToolkitDiagnosticDetail[] = [];
-    document.addEventListener(
-      EVENTS.diagnostic,
-      (event) => {
-        event.preventDefault();
-        diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-      },
-      { once: true },
-    );
+    const log = captureDiagnostics();
     vi.stubGlobal('scheduler', {
       postTask: () => Promise.reject(failure),
     });
@@ -179,7 +164,7 @@ describe('defaultScheduler.background', () => {
       vi.unstubAllGlobals();
     }
 
-    expect(diagnostics).toEqual([
+    expect(log.entries).toEqual([
       {
         severity: 'error',
         code: DIAGNOSTICS.scheduler.backgroundPostFailed,
@@ -187,6 +172,7 @@ describe('defaultScheduler.background', () => {
         error: failure,
       },
     ]);
+    log.stop();
   });
 });
 
@@ -210,15 +196,7 @@ describe('defaultScheduler.tick', () => {
 
   it('isolates a failed tick callback and continues the tick', async () => {
     const failure = new Error('tick failure');
-    const diagnostics: ToolkitDiagnosticDetail[] = [];
-    document.addEventListener(
-      EVENTS.diagnostic,
-      (event) => {
-        event.preventDefault();
-        diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-      },
-      { once: true },
-    );
+    const log = captureDiagnostics();
     let stopBroken = (): void => {};
     stopBroken = defaultScheduler.tick(() => {
       stopBroken();
@@ -234,7 +212,7 @@ describe('defaultScheduler.tick', () => {
     await nextFrame();
 
     expect(healthyCalls).toBe(1);
-    expect(diagnostics).toEqual([
+    expect(log.entries).toEqual([
       {
         severity: 'error',
         code: DIAGNOSTICS.callback.schedulerTickFailed,
@@ -242,6 +220,7 @@ describe('defaultScheduler.tick', () => {
         error: failure,
       },
     ]);
+    log.stop();
   });
 
   it('reports the time elapsed since the previous tick', async () => {
diff --git a/packages/v4/src/services/mixin.spec.ts b/packages/v4/src/services/mixin.spec.ts
index 8950030af..07286f7a8 100644
--- a/packages/v4/src/services/mixin.spec.ts
+++ b/packages/v4/src/services/mixin.spec.ts
@@ -2,7 +2,13 @@ import { afterEach, describe, expect, it } from 'vitest';
 import { Base } from '../Base.js';
 import { getInstance } from '../instances.js';
 import { registerComponent } from '../registry.js';
-import { countRequestedFrames, frames, resetDom, settle } from '../test/index.js';
+import {
+  captureDiagnostics,
+  countRequestedFrames,
+  frames,
+  resetDom,
+  settle,
+} from '../test/index.js';
 import { useDrag, withDrag } from './drag.js';
 import { withRaf } from './raf.js';
 import { withResize } from './resize.js';
@@ -119,14 +125,9 @@ describe('service mixins', () => {
   });
 
   it('subscribes for a component whose `mounted()` threw, as its handlers stay bound', async () => {
-    const failures: string[] = [];
-    const onDiagnostic = (event: Event) => {
-      // Cancel the default sink: the failure is the point of the spec, and
-      // `reportError()` would surface it as an unhandled error in the run.
-      event.preventDefault();
-      failures.push((event as CustomEvent<{ code: string }>).detail.code);
-    };
-    document.addEventListener('js-toolkit:diagnostic', onDiagnostic);
+    // The capture cancels every event, which is what the failure below needs:
+    // uncancelled, `reportError()` would surface it as an unhandled error.
+    const log = captureDiagnostics();
 
     class Broken extends withRaf(Base) {
       static config = { name: 'Broken' };
@@ -151,12 +152,12 @@ describe('service mixins', () => {
     const instance = new Broken(el).$mount();
     await frames(3);
     el.click();
-    document.removeEventListener('js-toolkit:diagnostic', onDiagnostic);
+    log.stop();
 
     // `#guard()` reports the failure and the cycle continues: the component is
     // mounted, its handlers are bound and its option effects are live, so the
     // subscription belongs with them rather than being the one thing withheld.
-    expect(failures).toContain('component.lifecycle-failed');
+    expect(log.codes).toContain('component.lifecycle-failed');
     expect(instance.$isMounted).toBe(true);
     expect(instance.clicks).toBe(1);
     expect(instance.ticks).toBeGreaterThan(0);
@@ -467,12 +468,7 @@ describe('service mixins', () => {
 
 describe('a resolver which comes back with nothing', () => {
   it('reports it and starts no subscription, rather than observing a default', async () => {
-    const codes: string[] = [];
-    const onDiagnostic = (event: Event) => {
-      event.preventDefault();
-      codes.push((event as CustomEvent<{ code: string }>).detail.code);
-    };
-    document.addEventListener('js-toolkit:diagnostic', onDiagnostic);
+    const log = captureDiagnostics();
 
     class Renamed extends Base {
       static config = { name: 'RenamedTarget' };
@@ -489,9 +485,9 @@ describe('a resolver which comes back with nothing', () => {
 
     const instance = new Mixed(render()).$mount();
     await settle();
-    document.removeEventListener('js-toolkit:diagnostic', onDiagnostic);
+    log.stop();
 
-    expect(codes).toEqual(['service.missing-target']);
+    expect(log.codes).toEqual(['service.missing-target']);
     // `useResize()` defaults its target to the document element, so without
     // this the component would have observed the page and looked fine.
     expect(instance.calls).toBe(0);
@@ -504,12 +500,7 @@ describe('a resolver which comes back with nothing', () => {
   });
 
   it('leaves a service whose own target is nothing alone', () => {
-    const codes: string[] = [];
-    const onDiagnostic = (event: Event) => {
-      event.preventDefault();
-      codes.push((event as CustomEvent<{ code: string }>).detail.code);
-    };
-    document.addEventListener('js-toolkit:diagnostic', onDiagnostic);
+    const log = captureDiagnostics();
 
     // `withRaf` resolves no target by design, and that is the contract rather
     // than a mistake — only a caller's resolver can be wrong.
@@ -517,9 +508,9 @@ describe('a resolver which comes back with nothing', () => {
     // unmounts an instance whose element carries no matching token — which is
     // correct, and would take this instance with it.
     const instance = new Ticker(render()).$mount();
-    document.removeEventListener('js-toolkit:diagnostic', onDiagnostic);
+    log.stop();
 
-    expect(codes).toEqual([]);
+    expect(log.codes).toEqual([]);
     // It subscribed: this service observes nothing by design, which is the
     // contract rather than a mistake.
     expect(instance.$services.ticked.isActive).toBe(true);
diff --git a/packages/v4/src/services/mutation.spec.ts b/packages/v4/src/services/mutation.spec.ts
index 483beb8cc..08db4fa33 100644
--- a/packages/v4/src/services/mutation.spec.ts
+++ b/packages/v4/src/services/mutation.spec.ts
@@ -1,7 +1,6 @@
 import { afterEach, beforeEach, describe, expect, expectTypeOf, it } from 'vitest';
 import { Base } from '../Base.js';
 import { DIAGNOSTICS, type ToolkitDiagnosticDetail } from '../diagnostic-contract.js';
-import { EVENTS } from '../events.js';
 import {
   useMutation,
   withMutation,
@@ -11,6 +10,7 @@ import {
 } from './mutation.js';
 import type { Service } from './service.js';
 import type { Toggle } from './toggle.js';
+import { captureDiagnostics } from '../test/index.js';
 
 interface Observation {
   target: Node;
@@ -94,19 +94,15 @@ function recordFor(target: Node, type: MutationRecordType = 'childList'): Mutati
   };
 }
 
+/** {@link captureDiagnostics} bounded to one synchronous call. */
 function catchDiagnostics(run: () => void): ToolkitDiagnosticDetail[] {
-  const diagnostics: ToolkitDiagnosticDetail[] = [];
-  const onDiagnostic = (event: Event) => {
-    event.preventDefault();
-    diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-  };
-  document.addEventListener(EVENTS.diagnostic, onDiagnostic);
+  const log = captureDiagnostics();
   try {
     run();
   } finally {
-    document.removeEventListener(EVENTS.diagnostic, onDiagnostic);
+    log.stop();
   }
-  return diagnostics;
+  return log.entries;
 }
 
 function readonlyAssertions(props: MutationProps): void {
diff --git a/packages/v4/src/services/pointer.spec.ts b/packages/v4/src/services/pointer.spec.ts
index 791317342..f19941c27 100644
--- a/packages/v4/src/services/pointer.spec.ts
+++ b/packages/v4/src/services/pointer.spec.ts
@@ -10,6 +10,7 @@ import {
 } from './pointer.js';
 import type { Service } from './service.js';
 import type { Toggle } from './toggle.js';
+import { frames } from '../test/index.js';
 
 function snapshot(props: PointerProps) {
   return { ...props };
@@ -41,13 +42,6 @@ function countReads(el: Element) {
   return { box, reads: () => reads };
 }
 
-/** A `ResizeObserver` delivers after the frame's callbacks, so two frames is the safe wait. */
-async function frames(count = 2): Promise<void> {
-  for (let index = 0; index < count; index += 1) {
-    await new Promise<void>((resolve) => requestAnimationFrame(() => resolve()));
-  }
-}
-
 async function scrollWindowTo(y: number): Promise<void> {
   const scrolled = new Promise<void>((resolve) =>
     window.addEventListener('scroll', () => resolve(), { once: true }),
@@ -333,7 +327,8 @@ describe('usePointer(target)', () => {
     expect(seen.at(-1)?.relativeProgressX).toBeCloseTo(0.25, 5);
 
     el.style.width = '400px';
-    await frames();
+    // A `ResizeObserver` delivers after the frame's callbacks, so two frames.
+    await frames(2);
 
     move(150, 80);
     expect(seen.at(-1)?.relativeProgressX).toBeCloseTo(0.125, 5);
diff --git a/packages/v4/src/services/service.spec.ts b/packages/v4/src/services/service.spec.ts
index d290ebfaf..6e872953f 100644
--- a/packages/v4/src/services/service.spec.ts
+++ b/packages/v4/src/services/service.spec.ts
@@ -1,21 +1,17 @@
 import { describe, expect, it } from 'vitest';
 import { DIAGNOSTICS, type ToolkitDiagnosticDetail } from '../diagnostic-contract.js';
-import { EVENTS } from '../events.js';
 import { createService, perTarget } from './service.js';
+import { captureDiagnostics } from '../test/index.js';
 
+/** {@link captureDiagnostics} bounded to one synchronous call. */
 function catchDiagnostics(run: () => void): ToolkitDiagnosticDetail[] {
-  const diagnostics: ToolkitDiagnosticDetail[] = [];
-  const onDiagnostic = (event: Event) => {
-    event.preventDefault();
-    diagnostics.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-  };
-  document.addEventListener(EVENTS.diagnostic, onDiagnostic);
+  const log = captureDiagnostics();
   try {
     run();
   } finally {
-    document.removeEventListener(EVENTS.diagnostic, onDiagnostic);
+    log.stop();
   }
-  return diagnostics;
+  return log.entries;
 }
 
 describe('createService', () => {
diff --git a/packages/v4/src/storage/createStorage.spec.ts b/packages/v4/src/storage/createStorage.spec.ts
index d614b3190..2b4d350b2 100644
--- a/packages/v4/src/storage/createStorage.spec.ts
+++ b/packages/v4/src/storage/createStorage.spec.ts
@@ -2,24 +2,21 @@ import { afterEach, describe, expect, it, vi } from 'vitest';
 import type { ToolkitDiagnosticDetail } from '../diagnostic-contract.js';
 import { createStorage } from './createStorage.js';
 import { createFallbackProvider, createMemoryStorageProvider } from './providers.js';
+import { captureDiagnostics } from '../test/index.js';
 import type { StorageProvider } from './types.js';
 
 /** Collect diagnostics dispatched while `during` runs, and silence the default sink. */
-async function captureDiagnostics(
+/** {@link captureDiagnostics} bounded to one awaited call. */
+async function catchDiagnostics(
   during: () => Promise<void> | void,
 ): Promise<ToolkitDiagnosticDetail[]> {
-  const seen: ToolkitDiagnosticDetail[] = [];
-  const listener = (event: Event) => {
-    seen.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-    event.preventDefault();
-  };
-  document.addEventListener('js-toolkit:diagnostic', listener);
+  const log = captureDiagnostics();
   try {
     await during();
   } finally {
-    document.removeEventListener('js-toolkit:diagnostic', listener);
+    log.stop();
   }
-  return seen;
+  return log.entries;
 }
 
 describe('createStorage', () => {
@@ -106,7 +103,7 @@ describe('createStorage', () => {
     });
     const unsubscribe = storage.subscribe('theme', (value) => seen.push(value));
 
-    const diagnostics = await captureDiagnostics(() => storage.set('theme', 'dark'));
+    const diagnostics = await catchDiagnostics(() => storage.set('theme', 'dark'));
 
     expect(seen).toEqual(['dark']);
     expect(diagnostics.map((detail) => detail.code)).toEqual(['callback.signal-failed']);
@@ -120,7 +117,7 @@ describe('createStorage', () => {
     const storage = createStorage<{ theme: string }>({ provider });
 
     let value: string | undefined;
-    const diagnostics = await captureDiagnostics(() => {
+    const diagnostics = await catchDiagnostics(() => {
       value = storage.get('theme', 'light');
     });
 
@@ -134,7 +131,7 @@ describe('createStorage', () => {
     const circular: Record<string, unknown> = {};
     circular.self = circular;
 
-    const diagnostics = await captureDiagnostics(() => storage.set('loop', circular));
+    const diagnostics = await catchDiagnostics(() => storage.set('loop', circular));
 
     expect(diagnostics.map((detail) => detail.code)).toEqual(['storage.serialize-failed']);
     expect(provider.has('loop')).toBe(false);
diff --git a/packages/v4/src/storage/providers.spec.ts b/packages/v4/src/storage/providers.spec.ts
index 08cf4c7e7..f21ee0477 100644
--- a/packages/v4/src/storage/providers.spec.ts
+++ b/packages/v4/src/storage/providers.spec.ts
@@ -1,6 +1,5 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import { DIAGNOSTICS, type ToolkitDiagnosticDetail } from '../diagnostic-contract.js';
-import { EVENTS } from '../events.js';
 import {
   createUrlSearchParamsInHashProvider,
   createUrlSearchParamsProvider,
@@ -9,21 +8,17 @@ import {
   urlSearchParamsInHashProvider,
   urlSearchParamsProvider,
 } from './providers.js';
+import { captureDiagnostics } from '../test/index.js';
 
-/** Collect diagnostics dispatched while `during` runs, and silence the default sink. */
-function captureDiagnostics(during: () => void): ToolkitDiagnosticDetail[] {
-  const seen: ToolkitDiagnosticDetail[] = [];
-  const listener = (event: Event) => {
-    seen.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail);
-    event.preventDefault();
-  };
-  document.addEventListener(EVENTS.diagnostic, listener);
+/** {@link captureDiagnostics} bounded to one synchronous call. */
+function catchDiagnostics(during: () => void): ToolkitDiagnosticDetail[] {
+  const log = captureDiagnostics();
   try {
     during();
   } finally {
-    document.removeEventListener(EVENTS.diagnostic, listener);
+    log.stop();
   }
-  return seen;
+  return log.entries;
 }
 
 /**
@@ -88,7 +83,7 @@ describe.each(webStorages)('the $name provider', ({ name, provider }) => {
       throw quotaExceeded;
     });
 
-    const diagnostics = captureDiagnostics(() => {
+    const diagnostics = catchDiagnostics(() => {
       expect(() => provider.set('theme', 'dark')).not.toThrow();
     });
 
@@ -103,7 +98,7 @@ describe.each(webStorages)('the $name provider', ({ name, provider }) => {
     const results: unknown[] = [];
 
     try {
-      const diagnostics = captureDiagnostics(() => {
+      const diagnostics = catchDiagnostics(() => {
         expect(() => {
           results.push(provider.get('theme'));
           results.push(provider.has('theme'));
diff --git a/packages/v4/src/swap.spec.ts b/packages/v4/src/swap.spec.ts
index 2a68ef6a6..461993232 100644
--- a/packages/v4/src/swap.spec.ts
+++ b/packages/v4/src/swap.spec.ts
@@ -1,11 +1,10 @@
 import { afterEach, describe, expect, it } from 'vitest';
 import { Base } from './Base.js';
-import { DIAGNOSTICS, type ToolkitDiagnosticDetail } from './diagnostic-contract.js';
-import { EVENTS } from './events.js';
-import { INSTANCES } from './protocol-symbols.js';
+import { DIAGNOSTICS } from './diagnostic-contract.js';
+import { getInstance } from './instances.js';
 import { registerComponent } from './registry.js';
 import { SWAP_MODES, swap } from './swap.js';
-import { resetDom } from './test/index.js';
+import { captureDiagnostics, resetDom } from './test/index.js';
 
 let counter = 0;
 
@@ -235,7 +234,7 @@ describe('swap — component lifecycle', () => {
     });
 
     expect(log).toEqual(['mounted:first', 'mounted:second']);
-    expect(el.querySelector('#second')?.[INSTANCES]?.get(name)?.$isMounted).toBe(true);
+    expect(getInstance(el.querySelector('#second'), name)?.$isMounted).toBe(true);
 
     await swap(el, '<p>gone</p>');
 
@@ -260,7 +259,7 @@ describe('swap — component lifecycle', () => {
     const el = target();
     await swap(el, `<div id="kept" data-component="${name}">before</div>`);
     const inner = el.querySelector('#kept');
-    const instance = inner?.[INSTANCES]?.get(name);
+    const instance = getInstance(inner, name);
     expect(log).toEqual(['mounted']);
 
     await swap(el, `<div id="kept" data-component="${name}">after</div>`, {
@@ -268,7 +267,7 @@ describe('swap — component lifecycle', () => {
     });
 
     expect(el.querySelector('#kept')).toBe(inner);
-    expect(inner?.[INSTANCES]?.get(name)).toBe(instance);
+    expect(getInstance(inner, name)).toBe(instance);
     expect(inner?.textContent).toBe('after');
     expect(log).toEqual(['mounted']);
   });
@@ -306,25 +305,15 @@ describe('swap — the wrap seam', () => {
     await swap(el, `<div data-component="${name}"></div>`, {
       wrap: (mutate) => {
         mutate();
-        mountedDuringWrap = el.firstElementChild?.[INSTANCES]?.get(name)?.$isMounted;
+        mountedDuringWrap = getInstance(el.firstElementChild, name)?.$isMounted;
       },
     });
 
     expect(mountedDuringWrap).toBeFalsy();
-    expect(el.firstElementChild?.[INSTANCES]?.get(name)?.$isMounted).toBe(true);
+    expect(getInstance(el.firstElementChild, name)?.$isMounted).toBe(true);
   });
 });
 
-/** Collect diagnostic codes until the returned cleanup runs. */
-function captureDiagnostics(): { codes: string[]; release: () => void } {
-  const codes: string[] = [];
-  const listener = (event: Event) => {
-    codes.push((event as CustomEvent<ToolkitDiagnosticDetail>).detail.code);
-  };
-  document.addEventListener(EVENTS.diagnostic, listener);
-  return { codes, release: () => document.removeEventListener(EVENTS.diagnostic, listener) };
-}
-
 describe('swap — replacing the target itself', () => {
   it('replaces the target element, attributes included', async () => {
     const parent = target('<div id="here" class="old" data-keep="yes"><p>old</p></div>');
@@ -391,16 +380,16 @@ describe('swap — replacing the target itself', () => {
 
     await swap(el, `<div id="here" data-component="${name}"></div>`, { self: true });
 
-    expect(parent.firstElementChild?.[INSTANCES]?.get(name)?.$isMounted).toBe(true);
+    expect(getInstance(parent.firstElementChild, name)?.$isMounted).toBe(true);
   });
 
   it('warns and keeps its meaning when a mode adds to the children', async () => {
     const parent = target('<div id="here"><p>old</p></div>');
     const el = parent.firstElementChild as HTMLElement;
-    const { codes, release } = captureDiagnostics();
+    const { codes, stop } = captureDiagnostics();
 
     await swap(el, '<p>new</p>', { mode: SWAP_MODES.APPEND, self: true });
-    release();
+    stop();
 
     expect(codes).toEqual([DIAGNOSTICS.swap.selfIgnored]);
     expect(el.isConnected).toBe(true);
@@ -410,10 +399,10 @@ describe('swap — replacing the target itself', () => {
   it('warns when the content holds no element to replace the target with', async () => {
     const parent = target('<div id="here"><p>old</p></div>');
     const el = parent.firstElementChild as HTMLElement;
-    const { codes, release } = captureDiagnostics();
+    const { codes, stop } = captureDiagnostics();
 
     await swap(el, 'text only', { self: true });
-    release();
+    stop();
 
     expect(codes).toEqual([DIAGNOSTICS.swap.selfIgnored]);
     expect(el.isConnected).toBe(true);
diff --git a/packages/v4/src/viewTransition.spec.ts b/packages/v4/src/viewTransition.spec.ts
index 491db1ebb..ccbec6391 100644
--- a/packages/v4/src/viewTransition.spec.ts
+++ b/packages/v4/src/viewTransition.spec.ts
@@ -1,6 +1,6 @@
 import { afterEach, describe, expect, it, vi } from 'vitest';
 import { defaultScheduler, nextFrame } from './scheduler.js';
-import { resetDom } from './test/index.js';
+import { resetDom, waitFor } from './test/index.js';
 import { viewTransition, type ViewTransitionUpdate } from './viewTransition.js';
 
 let restoreStartViewTransition = () => {};
@@ -41,9 +41,7 @@ function deferred() {
 }
 
 async function waitForBatch(started: number[], length: number): Promise<void> {
-  for (let index = 0; index < 50 && started.length < length; index += 1) {
-    await Promise.resolve();
-  }
+  await waitFor(() => started.length >= length);
   expect(started).toHaveLength(length);
 }