fix(vcs): keep Git Log rows painted while dragging the scrollbar (#1914)

Dragging the Git Log scrollbar through a long history stuttered and showed
blank rows: the native scrollbar scrolls asynchronously in WebKit, so the
virtual list rendered rows a frame or more after the new offset was painted.

The list now uses a scrollbar drawn in the DOM (`Scrollbar` in `src/ui`) that
sets the offset from pointer events and notifies scroll listeners at once, so
the virtualizer renders the matching rows before the next paint. Rows are
keyed by slot so a large jump updates the mounted rows instead of rebuilding
them, overscan is lowered, and rows rendered mid-scroll draw their graph
without edge hit targets, which come back once scrolling stops.

Adds a `gitLogScrollbar` perf scenario with budgets.

e2e/gitLogScrollbar.spec.ts

@@ -0,0 +1,106 @@
+import { expect, stubDesktop, test } from "./fixtures";
+
+test("keeps a wide Git graph filled during scrollbar drags and its edges interactive afterwards", async ({
+  workbench: page,
+}) => {
+  await stubDesktop(page);
+  await page.evaluate(async () => {
+    const root = "/dev/acme-api";
+    const count = 3000;
+    const id = (i: number) => i.toString(16).padStart(40, "0");
+    const commits = Array.from({ length: count }, (_, i) => ({
+      id: id(i),
+      shortId: id(i).slice(-8),
+      parents: i + 18 < count ? [id(i + 18)] : [],
+      refs: i === 0 ? ["HEAD", "main"] : [],
+      author: "Author",
+      email: "[email protected]",
+      timeMs: count - i,
+      summary: `Commit ${i}`,
+      message: `Commit ${i}`,
+    }));
+    const h = window.__ipc.handlers;
+    h.vcs_log = ({ query }) => {
+      const { skip, limit, range } = query as {
+        skip: number;
+        limit: number;
+        range?: string;
+      };
+      return range ? [] : commits.slice(skip, limit ? skip + limit : undefined);
+    };
+    h.vcs_log_refs = () => ({});
+    h.vcs_branches = () => [];
+    h.vcs_branch_info = () => ({
+      head: "main",
+      headCommit: id(0),
+      upstream: null,
+      ahead: 0,
+      behind: 0,
+      operation: null,
+    });
+    h.vcs_config_get = () => null;
+    h.vcs_commit_files = () => [];
+    h.vcs_log_details_batch = ({ revisions }) =>
+      (revisions as string[]).map((revision) => ({
+        message: `Commit ${parseInt(revision, 16)}`,
+        containingBranches: [],
+        signature: "N",
+        signer: "",
+        key: "",
+        fingerprint: "",
+      }));
+    const storeModule = "/src/features/vcs/store.ts";
+    const { useVcs } = await import(/* @vite-ignore */ storeModule);
+    useVcs.setState({
+      available: true,
+      root,
+      repo: root,
+      roots: [root],
+      branchesLoaded: true,
+    });
+    const contextModule = "/src/platform/context.ts";
+    const { setContext } = await import(/* @vite-ignore */ contextModule);
+    setContext("vcsAvailable", true);
+    const actionsModule = "/src/platform/actions.ts";
+    const { executeAction } = await import(/* @vite-ignore */ actionsModule);
+    await executeAction("vcs.showLog");
+  });
+  const list = page.getByRole("listbox", { name: "Commits" });
+  await expect(list.getByRole("option").first()).toBeVisible();
+  await page.evaluate(async () => {
+    const module = "/src/features/vcs/logStore.ts";
+    const { useGitLog, loadMore } = await import(/* @vite-ignore */ module);
+    while (!useGitLog.getState().done) await loadMore();
+  });
+
+  const box = (await list.boundingBox())!;
+  const x = box.x + box.width - 5;
+  await page.mouse.move(x, box.y + 8);
+  await page.mouse.down();
+  try {
+    for (const fraction of [0.2, 0.7, 0.4, 0.85, 0.3]) {
+      await page.mouse.move(x, box.y + box.height * fraction, { steps: 8 });
+      expect(
+        await list.evaluate((element) => {
+          const top = element.scrollTop;
+          const positions = [...element.querySelectorAll<HTMLElement>('[role="option"]')].map((row) => row.offsetTop);
+          return {
+            scrolling: top > 0,
+            covered: Math.min(...positions) <= top && Math.max(...positions) + 26 >= top + element.clientHeight,
+          };
+        }),
+      ).toEqual({ scrolling: true, covered: true });
+    }
+  } finally {
+    await page.mouse.up();
+  }
+
+  const index = await list.evaluate((element) => Math.ceil(element.scrollTop / 26));
+  const row = list.locator(`[id="commit-log-${index}"]`);
+  await row.click();
+  await expect(row).toHaveAttribute("aria-selected", "true");
+  await list.press("ArrowDown");
+  await expect(list.locator(`[id="commit-log-${index + 1}"]`)).toHaveAttribute("aria-selected", "true");
+  // Rows drawn mid-drag get their edge hit targets back once scrolling stops.
+  await expect(row.getByRole("button", { name: "Collapse this fragment" }).first()).toBeAttached();
+});

perf/budgets.json

@@ -51,6 +51,9 @@
     "gitLog.tbtMs": 50,
     "gitLog.arrowKeyP95Ms": 20,
     "gitLog.arrowKeyMissedFramePct": 5,
+    "gitLogScrollbar.frameP95Ms": 17.5,
+    "gitLogScrollbar.droppedFramesPct": 2,
+    "gitLogScrollbar.tbtMs": 50,
 
     "resizePanels.droppedFramesPct": 2,
     "resizePanels.bottomDroppedFramesPct": 2,

perf/scenarios.ts

@@ -316,6 +316,52 @@ const gitLog: Scenario = {
   },
 };
 
+/** Scrollbar-sized jumps across a loaded graph, rather than incremental wheel scrolling. */
+const gitLogScrollbar: Scenario = {
+  id: "gitLogScrollbar",
+  title: "Drag through a loaded 6k-commit Git Log",
+  async run(h) {
+    await h.page.evaluate(async () => {
+      const probe = (
+        window as never as {
+          __openideaPerf: {
+            fakeGitLog(n: number): void;
+            runAction(id: string): Promise<boolean>;
+          };
+        }
+      ).__openideaPerf;
+      probe.fakeGitLog(6000);
+      await probe.runAction("vcs.showLog");
+    });
+    const list = h.page.getByRole("listbox", { name: "Commits" });
+    await list.waitFor();
+    await list.evaluate(async (element) => {
+      for (let i = 0; i < 300 && element.scrollHeight < 6000 * 26; i++) {
+        element.scrollTop = element.scrollHeight;
+        await new Promise(requestAnimationFrame);
+      }
+      if (element.scrollHeight < 6000 * 26) throw new Error("Git Log did not finish loading");
+      element.scrollTop = 0;
+    });
+    await h.page.waitForTimeout(200);
+    const refresh = await h.refreshMs();
+    const t0 = await h.now();
+    await h.inPage((p) => p.startFrames());
+    await list.evaluate(async (element) => {
+      const max = element.scrollHeight - element.clientHeight;
+      for (let i = 0; i < 180; i++) {
+        element.scrollTop = ((i < 90 ? i : 180 - i) / 90) * max;
+        await new Promise(requestAnimationFrame);
+      }
+    });
+    const frames = await h.inPage((p) => p.stopFrames());
+    return {
+      ...frameSummary(frames, refresh),
+      tbtMs: totalBlockingTime(await h.longTasksSince(t0)),
+    };
+  },
+};
+
 /**
  * Dragging the splitters with an editor, the Project tree and the Git Log
  * open: every pointer move resizes the islands, so each frame relays them out.
@@ -555,6 +601,7 @@ export const SCENARIOS: Scenario[] = [
   completion,
   consoleOutput,
   gitLog,
+  gitLogScrollbar,
   resizePanels,
   keyRepeat,
   scroll,

src/features/vcs/GitLog.tsx

@@ -6,6 +6,7 @@ import { branchesByRepo, commitKey, commitRepo, graphCommit, type RepoCommit } f
 import { IssueText } from "./issueLinks";
 import { BranchFilterPopup } from "./BranchFilterPopup";
 import {
+  Fragment,
   memo,
   useCallback,
   useContext,
@@ -43,7 +44,9 @@ import {
   type TreeHandle,
   T,
   copyText,
+  hideNativeScrollbar,
   lazyComponent,
+  Scrollbar,
 } from "@/ui";
 import { hasPrimaryModifier } from "@/platform/keymap";
 import { laneColor, reachableFrom, type GraphRow, type GraphSegment } from "./graph";
@@ -1398,7 +1401,7 @@ function CommitList() {
     count: commits.length + (finished ? 0 : 1),
     getScrollElement: () => scrollRef.current,
     estimateSize: () => ROW,
-    overscan: 20,
+    overscan: 8,
     scrollMargin: ROW,
   });
   const items = virtualizer.getVirtualItems();
@@ -1410,7 +1413,10 @@ function CommitList() {
     if (!finished && !loading && last && last.index >= commits.length - PAGE) void loadMore(scope ?? undefined);
   }, [last, commits.length, finished, loading, active, scope]);
 
-  const selectedIndex = selected ? commits.findIndex((c) => commitKey(c) === selected) : -1;
+  const selectedIndex = useMemo(
+    () => (selected ? commits.findIndex((c) => commitKey(c) === selected) : -1),
+    [commits, selected],
+  );
   useLayoutEffect(() => {
     if (selectedIndex >= 0) virtualizer.scrollToIndex(selectedIndex, { align: "center" });
     // Only on explicit reveal requests.
@@ -1669,9 +1675,10 @@ function CommitList() {
           {error}
         </div>
       )}
+      <Scrollbar target={scrollRef} contentHeight={virtualizer.getTotalSize()} />
       <div
         ref={scrollRef}
-        className={styles.list}
+        className={clsx(styles.list, hideNativeScrollbar)}
         role="listbox"
         aria-label={tx("Commits")}
         aria-multiselectable
@@ -1730,7 +1737,9 @@ function CommitList() {
               );
             return (
               <CommitRow
-                key={commitKey(c)}
+                // Keyed by slot, not commit: a scrollbar drag that jumps past every mounted
+                // row then updates the same DOM nodes instead of rebuilding them.
+                key={v.index % items.length}
                 id={`commit-${scope ?? "log"}-${v.index}`}
                 commit={c}
                 graph={rows[v.index]}
@@ -1751,6 +1760,7 @@ function CommitList() {
                 targetTitle={targetTitle}
                 columns={columns}
                 bookmarked={!!bookmarks[commitKey(c)]}
+                interactive={!virtualizer.isScrolling}
               />
             );
           })}
@@ -1876,7 +1886,7 @@ function rowRemoteNames(own: readonly string[] | undefined, shared: ReadonlySet<
   return set;
 }
 
-const CommitRow = memo(function CommitRow({
+function CommitRowView({
   id,
   commit: c,
   graph,
@@ -1897,6 +1907,7 @@ const CommitRow = memo(function CommitRow({
   targetTitle,
   columns,
   bookmarked,
+  interactive,
 }: {
   id: string;
   commit: Commit;
@@ -1918,6 +1929,8 @@ const CommitRow = memo(function CommitRow({
   targetTitle: (index: number, key?: string) => string;
   columns: LogColumn[];
   bookmarked: boolean;
+  /** False for rows rendered mid-scroll: their graph is drawn without hit targets. */
+  interactive: boolean;
 }) {
   const merge = opts.highlightMerges && c.parents.length > 1;
   const subjectRef = useRef<HTMLSpanElement>(null);
@@ -1981,6 +1994,7 @@ const CommitRow = memo(function CommitRow({
             targetTitle={targetTitle}
             onFragment={(target, expand, source) => onFragment(source ?? commitKey(c), target, expand)}
             onNode={() => onFragment(commitKey(c), commitKey(c), false)}
+            interactive={interactive}
           />
         )}
         {bookmarked && <Icon name="bookmark" size={12} />}
@@ -2029,6 +2043,16 @@ const CommitRow = memo(function CommitRow({
       ))}
     </div>
   );
+}
+
+// A row that is already interactive stays so when scrolling starts; only rows
+// whose content changes mid-scroll are drawn without hit targets.
+const CommitRow = memo(CommitRowView, (prev, next) => {
+  for (const key in next) {
+    const k = key as keyof typeof next;
+    if (prev[k] !== next[k] && !(k === "interactive" && prev.interactive)) return false;
+  }
+  return true;
 });
 
 function x(lane: number) {
@@ -2051,6 +2075,7 @@ export function GraphCell({
   onFragment,
   targetTitle,
   onNode,
+  interactive = true,
 }: {
   row: GraphRow;
   head?: boolean;
@@ -2062,9 +2087,27 @@ export function GraphCell({
   onFragment?: (target: string, expand: boolean, source?: string) => void;
   targetTitle?: (index: number, key?: string) => string;
   onNode?: () => void;
+  interactive?: boolean;
 }) {
   const cy = ROW / 2;
   const draw = (s: GraphSegment, i: number, up: boolean) => {
+    const fromX = x(s.fromX ?? s.from);
+    const toX = x(s.toX ?? s.to);
+    const path =
+      seg(fromX, up ? 0 : cy, toX, up ? cy : ROW) +
+      (s.arrow ? arrowHead(up ? fromX : toX, up ? 0 : ROW, up ? -1 : 1) : "");
+    const line = (
+      <path
+        key={interactive ? undefined : `${up ? "u" : "d"}${i}`}
+        d={path}
+        stroke={laneColor(s.color)}
+        strokeWidth={s.edge != null && (activeColor === s.edge || highlighted?.has(s.edge)) ? 2.8 : 1.4}
+        pointerEvents="none"
+        strokeLinecap={s.dashed ? "butt" : "square"}
+        strokeDasharray={s.dashed ? "2 2" : undefined}
+      />
+    );
+    if (!interactive) return line;
     const target = (up ? row.upTargets : row.downTargets)?.[i];
     const canFragment =
       !!target &&
@@ -2086,50 +2129,34 @@ export function GraphCell({
       if (canNavigate) onNavigate?.(s.targetRow!, target);
       else if (canFragment) onFragment?.(target!, !!s.dashed, s.source);
     };
-    const fromX = x(s.fromX ?? s.from);
-    const toX = x(s.toX ?? s.to);
-    const path =
-      seg(fromX, up ? 0 : cy, toX, up ? cy : ROW) +
-      (s.arrow ? arrowHead(up ? fromX : toX, up ? 0 : ROW, up ? -1 : 1) : "");
     return (
-      <g
-        key={`${up ? "u" : "d"}${i}`}
-        role={actionable ? "button" : undefined}
-        tabIndex={actionable ? 0 : undefined}
-        aria-label={actionable ? label : undefined}
-        onMouseEnter={() => onHover?.(s.edge ?? `${s.source ?? ""}\0${target ?? ""}`)}
-        onMouseDown={(e) => e.stopPropagation()}
-        onClick={(e) => {
-          e.stopPropagation();
-          activate();
-        }}
-        onKeyDown={(e) => {
-          if (actionable && (e.key === "Enter" || e.key === " ")) {
-            e.preventDefault();
-            e.stopPropagation();
-            activate();
-          }
-        }}
-      >
-        <title>{label}</title>
-        <rect
-          x={Math.min(fromX, toX) - 5}
-          y={up ? 0 : cy}
-          width={Math.max(10, Math.abs(fromX - toX) + 10)}
-          height={cy}
-          fill="transparent"
-          pointerEvents="none"
-        />
-        <path d={path} stroke="transparent" strokeWidth={10} style={{ pointerEvents: "stroke" }} />
+      <Fragment key={`${up ? "u" : "d"}${i}`}>
         <path
           d={path}
-          stroke={laneColor(s.color)}
-          strokeWidth={s.edge != null && (activeColor === s.edge || highlighted?.has(s.edge)) ? 2.8 : 1.4}
-          pointerEvents="none"
-          strokeLinecap={s.dashed ? "butt" : "square"}
-          strokeDasharray={s.dashed ? "2 2" : undefined}
-        />
-      </g>
+          stroke="transparent"
+          strokeWidth={10}
+          style={{ pointerEvents: "stroke" }}
+          role={actionable ? "button" : undefined}
+          tabIndex={actionable ? 0 : undefined}
+          aria-label={actionable ? label : undefined}
+          onMouseEnter={() => onHover?.(s.edge ?? `${s.source ?? ""}\0${target ?? ""}`)}
+          onMouseDown={(e) => e.stopPropagation()}
+          onClick={(e) => {
+            e.stopPropagation();
+            activate();
+          }}
+          onKeyDown={(e) => {
+            if (actionable && (e.key === "Enter" || e.key === " ")) {
+              e.preventDefault();
+              e.stopPropagation();
+              activate();
+            }
+          }}
+        >
+          <title>{label}</title>
+        </path>
+        {line}
+      </Fragment>
     );
   };
   return (

src/ui/Scrollbar.module.css

@@ -0,0 +1,27 @@
+/* Same look as the global ::-webkit-scrollbar (global.css). */
+.track {
+  position: absolute;
+  top: 0;
+  right: 0;
+  width: 10px;
+  z-index: 1;
+}
+.thumb {
+  position: absolute;
+  left: 0;
+  right: 0;
+  top: 0;
+  border: 2px solid transparent;
+  border-radius: 6px;
+  background-clip: content-box;
+  background-color: var(--scrollbar-thumb);
+  will-change: transform;
+}
+.thumb:hover,
+.thumb[data-dragging] {
+  background-color: var(--scrollbar-thumb-hover);
+}
+/* For the scroller whose vertical scrollbar `Scrollbar` draws. */
+.hideNative::-webkit-scrollbar:vertical {
+  width: 0;
+}

src/ui/Scrollbar.test.tsx

@@ -0,0 +1,46 @@
+import { act, render } from "@testing-library/react";
+import { useRef } from "react";
+import { describe, expect, it, vi } from "vitest";
+import { Scrollbar } from "./Scrollbar";
+
+function Harness({ onScroll }: { onScroll: (top: number) => void }) {
+  const ref = useRef<HTMLDivElement>(null);
+  return (
+    <div style={{ position: "relative" }}>
+      <Scrollbar target={ref} contentHeight={1000} />
+      <div ref={ref} data-testid="scroller" onScroll={(e) => onScroll(e.currentTarget.scrollTop)} />
+    </div>
+  );
+}
+
+describe("Scrollbar", () => {
+  it("notifies scroll listeners synchronously while the thumb is dragged", () => {
+    const onScroll = vi.fn();
+    const { getByTestId, container } = render(<Harness onScroll={onScroll} />);
+    const scroller = getByTestId("scroller");
+    let top = 0;
+    Object.defineProperties(scroller, {
+      clientHeight: { value: 100 },
+      scrollHeight: { value: 1000 },
+      scrollTop: { get: () => top, set: (v: number) => (top = v) },
+    });
+    scroller.scrollTo = ((options: ScrollToOptions) => {
+      top = Math.max(0, Math.min(900, options.top ?? top));
+    }) as typeof scroller.scrollTo;
+    act(() => {
+      scroller.dispatchEvent(new Event("scroll"));
+    });
+    onScroll.mockClear();
+
+    const track = container.querySelector<HTMLElement>("[aria-hidden]")!;
+    const thumb = track.firstElementChild as HTMLElement;
+    track.setPointerCapture = () => {};
+    thumb.dispatchEvent(new PointerEvent("pointerdown", { bubbles: true, button: 0, clientY: 10 }));
+    track.dispatchEvent(new PointerEvent("pointermove", { bubbles: true, clientY: 46 }));
+
+    // A 32px thumb travels 68px of the track for 900px of content: 36px ≈ 476px.
+    expect(top).toBeCloseTo((36 / 68) * 900);
+    expect(onScroll).toHaveBeenCalledTimes(1);
+    expect(onScroll).toHaveBeenLastCalledWith(top);
+  });
+});

src/ui/Scrollbar.tsx

@@ -0,0 +1,111 @@
+import { useEffect, useLayoutEffect, useRef, type RefObject } from "react";
+import styles from "./Scrollbar.module.css";
+
+const MIN_THUMB = 32;
+
+/** Class for the scroller: hides its native vertical scrollbar, which `Scrollbar` replaces. */
+export const hideNativeScrollbar = styles.hideNative;
+
+/**
+ * A vertical scrollbar drawn in the DOM over `target` (place both in a
+ * `position: relative` box). Dragging it sets `scrollTop` from pointer events,
+ * so content rendered on scroll (virtual lists) lands in the same frame. The
+ * native one scrolls asynchronously in WebKit and shows blank rows until the
+ * page catches up.
+ *
+ * `contentHeight` re-measures when the scroller's content grows.
+ */
+export function Scrollbar({
+  target,
+  contentHeight,
+}: {
+  target: RefObject<HTMLElement | null>;
+  contentHeight?: number;
+}) {
+  const track = useRef<HTMLDivElement>(null);
+  const thumb = useRef<HTMLDivElement>(null);
+  const drag = useRef<{ y: number; top: number } | null>(null);
+  const geometry = useRef<Geometry | null>(null);
+
+  useLayoutEffect(() => {
+    geometry.current = layout(target.current, track.current, thumb.current);
+  }, [target, contentHeight]);
+  useEffect(() => {
+    const el = target.current;
+    if (!el) return;
+    const sync = () => {
+      geometry.current = layout(el, track.current, thumb.current);
+    };
+    el.addEventListener("scroll", sync, { passive: true });
+    const observer = new ResizeObserver(sync);
+    observer.observe(el);
+    return () => {
+      el.removeEventListener("scroll", sync);
+      observer.disconnect();
+    };
+  }, [target]);
+
+  return (
+    <div
+      ref={track}
+      className={styles.track}
+      aria-hidden
+      onPointerDown={(e) => {
+        const el = target.current;
+        if (!el || e.button !== 0) return;
+        e.preventDefault();
+        // A click on the track pages towards it, as the native scrollbar does.
+        if (e.target !== thumb.current) {
+          const { top, bottom } = thumb.current!.getBoundingClientRect();
+          if (e.clientY < top) scrollNow(el, el.scrollTop - el.clientHeight);
+          else if (e.clientY > bottom) scrollNow(el, el.scrollTop + el.clientHeight);
+          return;
+        }
+        drag.current = { y: e.clientY, top: el.scrollTop };
+        thumb.current!.dataset.dragging = "";
+        e.currentTarget.setPointerCapture(e.pointerId);
+      }}
+      onPointerMove={(e) => {
+        const el = target.current;
+        if (!el || !drag.current) return;
+        const g = geometry.current;
+        if (g) scrollNow(el, drag.current.top + ((e.clientY - drag.current.y) / g.range) * g.max);
+      }}
+      onLostPointerCapture={() => {
+        drag.current = null;
+        delete thumb.current?.dataset.dragging;
+      }}
+    >
+      <div ref={thumb} className={styles.thumb} />
+    </div>
+  );
+}
+
+type Geometry = { range: number; max: number };
+
+/** Positions the track over `el` and the thumb at its scroll offset; null when nothing scrolls. */
+function layout(el: HTMLElement | null, t: HTMLElement | null, th: HTMLElement | null): Geometry | null {
+  if (!el || !t || !th) return null;
+  const height = el.clientHeight;
+  const max = el.scrollHeight - height;
+  t.style.top = `${el.offsetTop}px`;
+  t.style.height = `${height}px`;
+  t.style.display = max > 0 ? "" : "none";
+  if (max <= 0) return null;
+  const size = Math.max(MIN_THUMB, (height * height) / el.scrollHeight);
+  const range = height - size;
+  th.style.height = `${size}px`;
+  th.style.transform = `translateY(${(Math.min(max, el.scrollTop) / max) * range}px)`;
+  return range > 0 ? { range, max } : null;
+}
+
+/**
+ * Scrolls `el` and tells its scroll listeners at once. WebKit fires `scroll` for
+ * a script scroll only at a later rendering step and may paint the new offset
+ * first, so a virtual list would show a frame of blank rows mid-drag.
+ */
+function scrollNow(el: HTMLElement, top: number) {
+  const before = el.scrollTop;
+  el.scrollTo({ top, behavior: "instant" });
+  if (el.scrollTop !== before) el.dispatchEvent(new Event("scroll"));
+}

src/ui/index.ts

@@ -25,6 +25,7 @@ export * from "./Popup";
 export * from "./Menu";
 export * from "./Dialog";
 export * from "./Splitter";
+export { Scrollbar, hideNativeScrollbar } from "./Scrollbar";
 export * from "./SourcePreview";
 export * from "./Feedback";
 export * from "./HighlightedText";