[diffs/edit]: keep virtual EOF caret anchored after line reinsertion

packages/diffs/src/editor/editor.ts

@@ -1336,12 +1336,7 @@ export class Editor<LAnnotation> implements DiffsEditor<LAnnotation> {
   }
 
   // Drop the DOM-geometry caches (memoized row elements, measured line Ys,
-  // last caret x). Wrap offsets are intentionally NOT cleared here: they
-  // depend only on line text, font metrics, and content width — not on the
-  // rendered rows — so they stay valid across render-range syncs and row
-  // rebuilds. The sites where those inputs actually change (text edits in
-  // #applyChange, width changes in #handleLayoutResize, font remeasure,
-  // document swaps, cleanUp) invalidate the wrap cache themselves.
+  // last caret x).
   #resetCache(): void {
     this.#lineYCache.clear();
     this.#lineElementsCache.clear();
@@ -3158,16 +3153,11 @@ export class Editor<LAnnotation> implements DiffsEditor<LAnnotation> {
       );
     }
 
-    // A diff re-renders its rows in place after the edits above: a unified diff
-    // rebuilds its content column on every edit (FileDiff.refreshDiffView swaps
-    // the column's innerHTML), and any diff rebuilds on a line-count change
-    // (applyDocumentChange -> full render). That detaches the line elements this
-    // editor memoized for caret/selection geometry (#lineYCache,
-    // #lastAccessedLineElement), so a detached row would measure offsetTop 0 and
-    // the caret would render at the top - the following #scrollToPrimaryCaret
-    // then scrolls the viewport there. Drop the geometry caches so the overlay
-    // re-measures against the freshly rebuilt rows and the caret stays put.
-    if (this.#isDiff && (this.#diffSyle === 'unified' || didLineCountChange)) {
+    // A line-count change can remove cached rows, while a unified diff rebuilds
+    // its content column on every edit. Either can detach the line elements
+    // memoized for caret/selection geometry, making offsetTop read as 0 and
+    // scrolling the caret to the top. Re-measure against the current rows.
+    if (didLineCountChange || (this.#isDiff && this.#diffSyle === 'unified')) {
       this.#resetCache();
     }
 

packages/diffs/test/editorVirtualizedEdit.test.ts

@@ -114,6 +114,68 @@ async function createWindowedEditor(
 }
 
 describe('Editor edits at the bottom of a virtualized window', () => {
+  test('keeps the last-line caret anchored after Enter, Delete, Enter', async () => {
+    const { cleanup, content, editor, fileContainer } =
+      await createWindowedEditor(1_000, makeRange(950, 50));
+    try {
+      Object.defineProperty(window.HTMLElement.prototype, 'offsetTop', {
+        configurable: true,
+        get(this: HTMLElement) {
+          const line = Number(this.dataset.line);
+          return this.isConnected && line > 0 ? line * 20 : 0;
+        },
+      });
+
+      editor.setSelections([
+        {
+          start: { line: 999, character: 9 },
+          end: { line: 999, character: 9 },
+          direction: 'none',
+        },
+      ]);
+      content.dispatchEvent(
+        new window.InputEvent('beforeinput', {
+          bubbles: true,
+          cancelable: true,
+          composed: true,
+          inputType: 'insertParagraph',
+        })
+      );
+      await wait(0);
+      const firstCaretTransform =
+        fileContainer.shadowRoot?.querySelector<HTMLElement>('[data-caret]')
+          ?.style.transform;
+      expect(firstCaretTransform).toBeDefined();
+
+      content.dispatchEvent(
+        new window.InputEvent('beforeinput', {
+          bubbles: true,
+          cancelable: true,
+          composed: true,
+          inputType: 'deleteContentBackward',
+        })
+      );
+      await wait(0);
+      content.dispatchEvent(
+        new window.InputEvent('beforeinput', {
+          bubbles: true,
+          cancelable: true,
+          composed: true,
+          inputType: 'insertParagraph',
+        })
+      );
+      await wait(0);
+
+      expect(editor.getState().selections?.at(-1)?.start.line).toBe(1_000);
+      expect(
+        fileContainer.shadowRoot?.querySelector<HTMLElement>('[data-caret]')
+          ?.style.transform
+      ).toBe(firstCaretTransform);
+    } finally {
+      cleanup();
+    }
+  });
+
   // A run of Enter presses at the window's bottom edge must keep rendering each
   // new line. The editor widens its render range when the caret reaches the
   // last rendered line; if that widened range is not persisted, the next edit