Fix fast Windows source scroll blanks

PUBLIC_SOURCE_MANIFEST.json

@@ -2,9 +2,9 @@
   "schemaVersion": 1,
   "license": "GPL-3.0-only",
   "historyPolicy": "sanitized-single-snapshot",
-  "generatedAt": "2026-08-13T05:21:04.140Z",
+  "generatedAt": "2026-08-13T07:09:25.850Z",
   "fileCount": 662,
-  "snapshotSha256": "2d3dd4f702abfab6cb7596cdc26ee9dc20e4c14f44cdffbe7e5215f45d2c6961",
+  "snapshotSha256": "0b83a8e782c8c7757047f97ef8a431156bbafbac2acf5645685fbe4d152d5e53",
   "files": [
     {
       "path": ".github/ISSUE_TEMPLATE/bug_report.yml",
@@ -1333,8 +1333,8 @@
     },
     {
       "path": "apps/windows/package.json",
-      "bytes": 1867,
-      "sha256": "1f69a2553ef25ebe45fd36b9b47aacee12cc79696232f080b969c37aa3a7e604"
+      "bytes": 1902,
+      "sha256": "bf447e38230931494f1cdae3829d75455c5384f38c461954aa8589f015951937"
     },
     {
       "path": "apps/windows/src/App.test.tsx",
@@ -2133,23 +2133,23 @@
     },
     {
       "path": "apps/windows/src/ui/components/SessionSourceEditor.test.tsx",
-      "bytes": 24331,
-      "sha256": "38b4f1810ceeea0c16c644c66503c8f58cd2071820659fe076da6655b3dbbaf5"
+      "bytes": 25296,
+      "sha256": "cedebb9dcb485d445723627bb9242f19be0d9aa0ee783b11c21f04588dd2a0b5"
     },
     {
       "path": "apps/windows/src/ui/components/SessionSourceEditor.tsx",
-      "bytes": 70499,
-      "sha256": "f7ccf688f05acd5f83cd03f4544b4d62f96b635e713894b0a2349b51b83bf102"
+      "bytes": 72334,
+      "sha256": "56a6593b2eba0181d29140ab1d2542f43bb052fb92b64d089a86e1e3ded1f1f2"
     },
     {
       "path": "apps/windows/src/ui/components/SourceTanStackLab.test.tsx",
-      "bytes": 2545,
-      "sha256": "b490b7e3041a94c09a1dca5c0124da4063eadc2ed8fd6087577efd27cbab6924"
+      "bytes": 2817,
+      "sha256": "f31dac0e71641bea8077e3800d61e574222765f6ec6ea909e4787d34686ed55b"
     },
     {
       "path": "apps/windows/src/ui/components/SourceTanStackLab.tsx",
-      "bytes": 4268,
-      "sha256": "481ea273b60ed9ab4c8ebfb9dbf308b99d8b716ae6f7671abc95ac718e2f1d3b"
+      "bytes": 4261,
+      "sha256": "697d5377b5c87770594d10a945439169a910f53f1def978163a38194387e8a57"
     },
     {
       "path": "apps/windows/src/ui/components/sessionSourceEditorModel.test.ts",
@@ -2333,13 +2333,13 @@
     },
     {
       "path": "package-lock.json",
-      "bytes": 354932,
-      "sha256": "f546ff664639e6445ee05ca3b00de206eb722d218cd9fba3ed7e206b5546613c"
+      "bytes": 354975,
+      "sha256": "efe35fef34a3de44e9bbf41aa6577c5bb2073b86609763e35538875311903d40"
     },
     {
       "path": "package.json",
-      "bytes": 8069,
-      "sha256": "5bcb28a9d95ee4a9e9ff7843e2aca4a0d1d265c490ca21182d4cd022beef0f4e"
+      "bytes": 8494,
+      "sha256": "35c59f95ac9a6e8b4a9cc7f8a125d1c6b038918b45d46ff8a6e65a863faad6c8"
     },
     {
       "path": "packages/core-server/package.json",
@@ -2998,8 +2998,8 @@
     },
     {
       "path": "test_tool/editor_ui_performance_lab.mjs",
-      "bytes": 143768,
-      "sha256": "7bb96b6d41a3e7d6c8724df7732bb44dc92ecd96d7a242cbe1eb10f17deaf54a"
+      "bytes": 152015,
+      "sha256": "88f97c2dffd82fe250630336edb9b5c289b9d81b14ffd25ebb683951fca1994d"
     },
     {
       "path": "test_tool/electron_actual_state_probe.mjs",
@@ -3008,8 +3008,8 @@
     },
     {
       "path": "test_tool/electron_app_smoke.mjs",
-      "bytes": 51960,
-      "sha256": "604a2c4db8323cf964ab9b6ab818c5fce09b39e6013b2bb13fcd9d4a4b035a7e"
+      "bytes": 52403,
+      "sha256": "7d30a27a98bb4d7ed50004d37d9d3ba3d92d6c47b8b6c3e3c9c50e2743523c5e"
     },
     {
       "path": "test_tool/export_public_source.mjs",

apps/windows/package.json

@@ -15,6 +15,7 @@
   "dependencies": {
     "@codemirror/lang-markdown": "^6.5.0",
     "@codemirror/state": "^6.7.0",
+    "@codemirror/view": "6.43.8",
     "@mathnotes/core-server": "file:../../packages/core-server",
     "@mathnotes/shared": "file:../../packages/shared",
     "@tanstack/react-virtual": "^3.14.6",

apps/windows/src/ui/components/SessionSourceEditor.test.tsx

@@ -7,6 +7,8 @@ import {
   buildPerformanceLabVirtualLayout,
   expandPerformanceLabVirtualRange,
   findPerformanceLabVirtualRange,
+  getTanStackSourceWindowingEffectiveMode,
+  getTanStackSourceWindowingFallbackReason,
   groupAssistantRemarksByBlockId,
   findEditorCaretTargetAtPoint,
   SessionSourceEditor,
@@ -233,6 +235,19 @@ describe("SessionSourceEditor", () => {
     expect(screen.queryByTestId("editor-context-menu")).toBeNull();
   });
 
+  it("keeps small sessions and sessions with a very long block in normal document flow", () => {
+    expect(getTanStackSourceWindowingFallbackReason(["one", "two"])).toBe("small-session");
+    expect(getTanStackSourceWindowingEffectiveMode(["one", "two"])).toBe("off");
+
+    const ordinaryBlocks = Array.from({ length: 13 }, (_, index) => `block ${index + 1}\nsecond line`);
+    expect(getTanStackSourceWindowingFallbackReason(ordinaryBlocks)).toBeNull();
+    expect(getTanStackSourceWindowingEffectiveMode(ordinaryBlocks)).toBe("tanstack-virtual");
+
+    const longBlock = Array.from({ length: 85 }, (_, index) => `line ${index + 1}`).join("\n");
+    expect(getTanStackSourceWindowingFallbackReason([...ordinaryBlocks.slice(0, 12), longBlock])).toBe("long-block");
+    expect(getTanStackSourceWindowingEffectiveMode([...ordinaryBlocks.slice(0, 12), longBlock])).toBe("off");
+  });
+
   it("sends the exact UTF-16 selection to the AI edit callback from the editor context menu", async () => {
     Range.prototype.getClientRects = () => [] as unknown as DOMRectList;
     Range.prototype.getBoundingClientRect = () =>

apps/windows/src/ui/components/SessionSourceEditor.tsx

@@ -179,6 +179,7 @@ type EditorStateSnapshot = {
 const performanceLabEditorViewLimit = 12;
 const performanceLabOverscanPx = 1200;
 const performanceLabVirtualSegmentSize = 12;
+const performanceLabLongSourceBlockLineLimit = 85;
 const performanceLabStorageKey = "mathnotes:editor-windowing-lab";
 const performanceLabSourceOverscanStorageKey = "mathnotes:source-overscan-lab";
 const performanceLabVirtualHeaderHeight = 39;
@@ -217,6 +218,34 @@ export function buildPerformanceLabVirtualLayout(markdowns: readonly string[]):
   return { offsets, sizes, totalSize };
 }
 
+export function getTanStackSourceWindowingFallbackReason(
+  markdowns: readonly string[]
+): "small-session" | "long-block" | null {
+  // A session with at most twelve blocks gains little from a second layer of
+  // virtualization because CodeMirror already virtualizes the lines inside
+  // each block. Keeping these editors in normal document flow also avoids
+  // competing viewport calculations during native scrollbar drags.
+  if (markdowns.length <= performanceLabEditorViewLimit) return "small-session";
+
+  for (const markdown of markdowns) {
+    let lineCount = 1;
+    for (let index = 0; index < markdown.length; index += 1) {
+      if (markdown.charCodeAt(index) !== 10) continue;
+      lineCount += 1;
+      if (lineCount >= performanceLabLongSourceBlockLineLimit) return "long-block";
+    }
+  }
+  return null;
+}
+
+export function getTanStackSourceWindowingEffectiveMode(
+  markdowns: readonly string[]
+): "off" | "tanstack-virtual" {
+  const fallbackReason = getTanStackSourceWindowingFallbackReason(markdowns);
+  if (fallbackReason) return "off";
+  return "tanstack-virtual";
+}
+
 export function findPerformanceLabVirtualRange({
   layout,
   scrollTop,
@@ -325,11 +354,7 @@ export function SessionSourceEditor({
   onRerecognizeBlockRequest,
   onTransferBlocksRequest
 }: SessionSourceEditorProps) {
-  const editorWindowingLabMode = getEditorWindowingPerformanceLabMode();
-  const dynamicEditorWindowingLab = editorWindowingLabMode === "dynamic";
-  const virtualBlockWindowingLab = editorWindowingLabMode === "virtual";
-  const tanStackSourceWindowingLab = editorWindowingLabMode === "tanstack-virtual";
-  const statefulEditorWindowingLab = dynamicEditorWindowingLab || virtualBlockWindowingLab || tanStackSourceWindowingLab;
+  const requestedEditorWindowingLabMode = getEditorWindowingPerformanceLabMode();
   const editorScrollerRef = useRef<HTMLDivElement>(null);
   const blockElementsRef = useRef(new Map<string, HTMLElement>());
   const editorViewsRef = useRef(new Map<string, EditorView>());
@@ -393,10 +418,26 @@ export function SessionSourceEditor({
     [document.markdownBlocks]
   );
   const remarksByBlockId = useMemo(() => groupAssistantRemarksByBlockId(assistantRemarks), [assistantRemarks]);
-  const virtualBlockLayout = useMemo(
-    () => buildPerformanceLabVirtualLayout(document.markdownBlocks.map((block) => markdownByBlockId[block.blockId] ?? "")),
+  const sourceMarkdowns = useMemo(
+    () => document.markdownBlocks.map((block) => markdownByBlockId[block.blockId] ?? ""),
     [document.markdownBlocks, markdownBlockIds, markdownByBlockId]
   );
+  const tanStackFallbackReason = requestedEditorWindowingLabMode === "tanstack-virtual"
+    ? getTanStackSourceWindowingFallbackReason(sourceMarkdowns)
+    : null;
+  const editorWindowingLabMode: EditorWindowingLabMode = requestedEditorWindowingLabMode === "tanstack-virtual"
+    ? getTanStackSourceWindowingEffectiveMode(sourceMarkdowns)
+    : requestedEditorWindowingLabMode;
+  const dynamicEditorWindowingLab = editorWindowingLabMode === "dynamic";
+  const virtualBlockWindowingLab = editorWindowingLabMode === "virtual";
+  const tanStackSourceWindowingLab = editorWindowingLabMode === "tanstack-virtual";
+  const statefulEditorWindowingLab = dynamicEditorWindowingLab ||
+    virtualBlockWindowingLab ||
+    requestedEditorWindowingLabMode === "tanstack-virtual";
+  const virtualBlockLayout = useMemo(
+    () => buildPerformanceLabVirtualLayout(sourceMarkdowns),
+    [sourceMarkdowns]
+  );
 
   useEffect(() => {
     // A local CodeMirror edit is projected to the aggregate source on the next frame.
@@ -1005,6 +1046,8 @@ export function SessionSourceEditor({
     <div
       className="session-source-editor source-block-shell"
       data-editor-windowing-lab={editorWindowingLabMode}
+      data-editor-windowing-requested={requestedEditorWindowingLabMode}
+      data-editor-windowing-fallback={tanStackFallbackReason ?? undefined}
       data-testid="session-source-editor"
       ref={editorScrollerRef}
     >
@@ -1206,6 +1249,7 @@ function SourceBlockSection({
           ].filter(Boolean).join(" ")}
           data-testid="source-block"
           data-block-id={block.blockId}
+          data-locked={block.locked ? "true" : "false"}
           data-locating-nonce={locatingNonce || undefined}
           data-source={block.sourceId}
           data-index={virtualIndex}

apps/windows/src/ui/components/SourceTanStackLab.test.tsx

@@ -1,11 +1,11 @@
 import { render, waitFor } from "@testing-library/react";
-import { createRef } from "react";
+import { createRef, type CSSProperties } from "react";
 import { describe, expect, it, vi } from "vitest";
 import { SourceTanStackLab } from "./SourceTanStackLab";
 
 vi.mock("@tanstack/react-virtual", () => ({
   useVirtualizer: () => ({
-    getVirtualItems: () => [{ index: 0, start: 0 }],
+    getVirtualItems: () => [{ index: 0, start: 37 }],
     getTotalSize: () => 100,
     itemSizeCache: new Map(),
     measureElement: () => undefined,
@@ -73,5 +73,12 @@ describe("SourceTanStackLab preview locate handshake", () => {
       </div>
     );
     expect(renderItem).toHaveBeenCalledWith(block, 0, expect.any(Function), expect.any(Object));
+    const style = (renderItem.mock.calls as unknown as Array<[unknown, number, unknown, CSSProperties]>)[0]?.[3];
+    expect(style).toMatchObject({
+      position: "absolute",
+      top: 0,
+      transform: "translateY(37px)",
+      width: "100%"
+    });
   });
 });

package.json

@@ -77,7 +77,9 @@
     "test:editor-performance": "node test_tool/editor_performance_smoke.mjs",
     "test:editor-performance:lab": "node test_tool/editor_performance_lab.mjs",
     "test:editor-performance:ui-lab": "node test_tool/editor_ui_performance_lab.mjs",
-    "test:windows-scroll-blank": "node test_tool/editor_ui_performance_lab.mjs --experiment=scroll-blank --shapes=80 --fixture-profile=heterogeneous --product-defaults=1 --output=output/performance/windows-scroll-blank-hotfix.json",
+    "test:windows-scroll-blank": "npm run test:windows-scroll-blank:long && npm run test:windows-scroll-blank:many",
+    "test:windows-scroll-blank:long": "node test_tool/editor_ui_performance_lab.mjs --experiment=scroll-blank --shapes=2 --fixture-profile=single-long --product-defaults=1 --scroll-direction-ms=200 --output=output/performance/windows-fast-scroll-single-long-v2.json",
+    "test:windows-scroll-blank:many": "node test_tool/editor_ui_performance_lab.mjs --experiment=scroll-blank --shapes=80 --fixture-profile=heterogeneous --product-defaults=1 --scroll-direction-ms=300 --output=output/performance/windows-fast-scroll-heterogeneous-v2.json",
     "test:prototype": "node test_tool/prototype_smoke.mjs http://127.0.0.1:4173/",
     "test:windows-smoke": "node test_tool/windows_app_smoke.mjs http://127.0.0.1:5173/",
     "test:electron-smoke": "node test_tool/electron_app_smoke.mjs",