Fix ComputedValue fails to trigger onBecomeObservable when used in action -> autorun (mobxjs#4700)

* fix: observe computed dependencies gained in actions

* fix: avoid hooks for temporary computed observers

* test: simplify computed lifecycle regression

Author: kubk

.changeset/observed-computeds-track-action-dependencies.md

@@ -0,0 +1,5 @@
+---
+"mobx": patch
+---
+
+Fix `onBecomeObserved` not firing for dependencies gained when an observed computed is recomputed inside an action.

packages/mobx/__tests__/base/become-observed.ts

@@ -615,3 +615,43 @@ test("onBecomeObserved cascades through a chain of cached computeds #4547", () =
     disposeAutorun!()
     expect(events).toEqual(["BO", "BUO"])
 })
+
+test("onBecomeObserved fires for a dependency gained by an observed computed in an action #3674", () => {
+    const events: string[] = []
+    const enabled = observable.box(false)
+    const resource = observable.box(1)
+    onBecomeObserved(resource, () => events.push("BO"))
+    onBecomeUnobserved(resource, () => events.push("BUO"))
+    const derived = computed(() => (enabled.get() ? resource.get() : null))
+    const disposeAutorun = autorun(() => {
+        derived.get()
+    })
+
+    runInAction(() => {
+        enabled.set(true)
+        derived.get()
+    })
+
+    expect(events).toEqual(["BO"])
+
+    disposeAutorun()
+    expect(events).toEqual(["BO", "BUO"])
+})
+
+test("temporary computed observers do not activate lifecycle hooks", () => {
+    const events: string[] = []
+    const enabled = observable.box(false)
+    const resource = observable.box(1)
+    onBecomeObserved(resource, () => events.push("BO"))
+    onBecomeUnobserved(resource, () => events.push("BUO"))
+    const inner = computed(() => (enabled.get() ? resource.get() : null))
+    const outer = computed(() => inner.get())
+
+    runInAction(() => {
+        outer.get()
+        enabled.set(true)
+        inner.get()
+    })
+
+    expect(events).toEqual([])
+})

packages/mobx/src/core/computedvalue.ts

@@ -216,7 +216,10 @@ export class ComputedValue<T> implements IObservable, IComputedValue<T>, IDeriva
             reportObserved(this)
             if (shouldCompute(this)) {
                 let prevTrackingContext = globalState.trackingContext
-                if (this.keepAlive_ && !prevTrackingContext) {
+                if (!prevTrackingContext && (this.keepAlive_ || this.isBeingObserved)) {
+                    // An observed or keep-alive computed can be recomputed by an untracked
+                    // read, for example from inside an action. Its dependencies are still
+                    // transitively observed and must fire their lifecycle hooks
                     globalState.trackingContext = this
                 }
                 if (this.trackAndCompute()) {