fix(ui): use full agent-note set for section geometry measurement (#243)

Co-authored-by: Ben Vinegar <ben@benv.ca>

Author: aldevv

src/ui/components/panes/DiffPane.tsx

@@ -442,11 +442,18 @@ export function DiffPane({
     return next;
   }, [allAgentNotesByFile, selectedFileId, showAgentNotes, visibleViewportFileIds]);
 
+  // Measure with the *full* set of agent notes per file, not just the visible-viewport set.
+  // The visible set is correct for rendering (skip painting cards on off-screen files), but
+  // using it here makes total content height fluctuate with scroll position: as a file with
+  // notes leaves the viewport, its measurement shrinks back to the no-notes baseline, which
+  // shrinks `totalContentHeight`, which tightens `clampReviewScrollTop`'s ceiling, which
+  // snaps the viewport upward by the height of the off-top note rows. Always include notes
+  // in geometry for stable bottom-edge clamping.
   const sectionGeometry = useMemo(
     () =>
       files.map((file, index) => {
-        const visibleNotes = visibleAgentNotesByFile.get(file.id) ?? EMPTY_VISIBLE_AGENT_NOTES;
-        if (visibleNotes.length === 0) {
+        const notes = allAgentNotesByFile.get(file.id) ?? EMPTY_VISIBLE_AGENT_NOTES;
+        if (notes.length === 0) {
           return baseSectionGeometry[index]!;
         }
 
@@ -455,21 +462,21 @@ export function DiffPane({
           layout,
           showHunkHeaders,
           theme,
-          visibleNotes,
+          notes,
           diffContentWidth,
           showLineNumbers,
           wrapLines,
         );
       }),
     [
+      allAgentNotesByFile,
       baseSectionGeometry,
       diffContentWidth,
       files,
       layout,
       showHunkHeaders,
       showLineNumbers,
       theme,
-      visibleAgentNotesByFile,
       wrapLines,
     ],
   );

src/ui/components/ui-components.test.tsx

@@ -856,6 +856,64 @@ describe("UI components", () => {
     }
   });
 
+  test("DiffPane keeps bottom scroll stable when offscreen agent notes are windowed out", async () => {
+    const theme = resolveTheme("midnight", null);
+    const firstFile = createTallDiffFile("first", "first.ts", 18);
+    firstFile.agent = {
+      path: firstFile.path,
+      summary: "first.ts note",
+      annotations: [
+        {
+          newRange: [2, 2],
+          summary: "Offscreen note should still reserve geometry at EOF.",
+          rationale:
+            "If measurement drops this note after first.ts leaves the viewport, max scroll shrinks.",
+        },
+      ],
+    };
+    const files = [firstFile, createTallDiffFile("last", "last.ts", 24)];
+    const scrollRef = createRef<ScrollBoxRenderable>();
+    const props = createDiffPaneProps(files, theme, {
+      diffContentWidth: 88,
+      headerLabelWidth: 48,
+      headerStatsWidth: 16,
+      scrollRef,
+      selectedFileId: undefined,
+      separatorWidth: 84,
+      showAgentNotes: true,
+      width: 92,
+    });
+    const setup = await testRender(<DiffPane {...props} />, {
+      width: 96,
+      height: 10,
+    });
+
+    try {
+      await settleDiffPane(setup);
+
+      let bottomScrollTop = 0;
+      await act(async () => {
+        scrollRef.current?.scrollTo(1_000_000);
+        bottomScrollTop = scrollRef.current?.scrollTop ?? 0;
+      });
+      expect(bottomScrollTop).toBeGreaterThan(0);
+
+      await settleDiffPane(setup);
+      expect(scrollRef.current?.scrollTop ?? 0).toBe(bottomScrollTop);
+
+      await act(async () => {
+        scrollRef.current?.scrollTo(bottomScrollTop + 1);
+      });
+      await settleDiffPane(setup);
+
+      expect(scrollRef.current?.scrollTop ?? 0).toBe(bottomScrollTop);
+    } finally {
+      await act(async () => {
+        setup.renderer.destroy();
+      });
+    }
+  });
+
   test("DiffPane lets manual scrolling move away from a bottom-clamped file-top alignment", async () => {
     const theme = resolveTheme("midnight", null);
     const files = [