fix(painter-dom): keep the empty-line caret on the start of RTL lines

packages/layout-engine/painters/dom/src/index.test.ts

@@ -2669,7 +2669,7 @@ describe('DomPainter', () => {
     painter.paint(emptyLayout, mount);
 
     const line = mount.querySelector('.superdoc-line');
-    expect(line?.textContent).toBe('\u00A0');
+    expect(line?.textContent).toBe('\u200B');
   });
 
   it('paints empty-line caret targets with the insertion run typography', () => {

packages/layout-engine/painters/dom/src/runs/render-line.test.ts

@@ -530,3 +530,51 @@ describe('renderLine inline boxes', () => {
     expect(cleared.textContent).toBe('beforeboxedafter');
   });
 });
+
+describe('renderLine empty-line placeholder', () => {
+  const emptyLine = (): Line => ({
+    fromRun: 0,
+    fromChar: 0,
+    toRun: 0,
+    toChar: 0,
+    width: 0,
+    maxWidth: 200,
+    ascent: 12,
+    descent: 4,
+    lineHeight: 18,
+    segments: [],
+  });
+
+  const emptyBlock = (attrs: ParagraphBlock['attrs'] = {}): ParagraphBlock => ({
+    kind: 'paragraph',
+    id: 'empty-block',
+    attrs,
+    runs: [{ kind: 'text', text: '', fontFamily: 'Arial', fontSize: 16, pmStart: 1, pmEnd: 1 }],
+  });
+
+  it('fills the placeholder with a zero-width space so it takes no advance', () => {
+    const lineEl = renderLine({
+      block: emptyBlock(),
+      line: emptyLine(),
+      context: { pageNumber: 1, totalPages: 1, section: 'body' },
+      runContext: makeRunContext(),
+    });
+
+    const placeholder = lineEl.querySelector<HTMLElement>('.superdoc-empty-run');
+    expect(placeholder?.textContent).toBe('\u200B');
+    expect(placeholder?.dataset.pmStart).toBe('1');
+    expect(placeholder?.dataset.pmEnd).toBe('1');
+  });
+
+  it('keeps the placeholder zero-advance on an RTL line', () => {
+    const lineEl = renderLine({
+      block: emptyBlock({ directionContext: { inlineDirection: 'rtl' } }),
+      line: emptyLine(),
+      context: { pageNumber: 1, totalPages: 1, section: 'body' },
+      runContext: makeRunContext(),
+    });
+
+    expect(lineEl.getAttribute('dir')).toBe('rtl');
+    expect(lineEl.querySelector('.superdoc-empty-run')?.textContent).toBe('\u200B');
+  });
+});

packages/layout-engine/painters/dom/src/runs/render-line.ts

@@ -41,6 +41,20 @@ function isMinimalWordLayout(value: unknown): value is MinimalWordLayout {
   return isMinimalWordLayoutShared(value);
 }
 
+/**
+ * Filler for the placeholder span painted on an empty line.
+ *
+ * The span carries PM positions; it is not meant to occupy space, so its filler
+ * must take no advance width. A non-breaking space takes one. The caret on an
+ * empty line is drawn at the placeholder's left edge, so that width pushes the
+ * caret off the start of the line: invisible on an LTR line, where the left edge
+ * is the line start, but visible on an RTL line, which starts at the right edge.
+ *
+ * A zero-width space keeps the span's font metrics, and with them the caret's
+ * height, while contributing no advance.
+ */
+const EMPTY_LINE_PLACEHOLDER = '\u200B';
+
 const applyStyles = (el: HTMLElement, styles: Partial<CSSStyleDeclaration>): void => {
   Object.entries(styles).forEach(([key, value]) => {
     if (value != null && value !== '' && key in el.style) {
@@ -541,7 +555,7 @@ export const renderLine = ({
     } else {
       span.style.fontSize = `${line.lineHeight}px`;
     }
-    span.innerHTML = '&nbsp;';
+    span.textContent = EMPTY_LINE_PLACEHOLDER;
     el.appendChild(span);
   }