fix(studio): a timeline at 00:00 stays at 00:00 when the window resizes or the zoom is set (#4411)

The centre-anchor step ran on every scale change, including a window resize and a zoom set by a host, and from scroll 0 it always moved the view right, hiding 00:00 under the track header. A view at the start now stays there; a scrolled view still keeps its centre time.

packages/studio/src/player/components/useTimelinePlayhead.test.tsx

@@ -0,0 +1,65 @@
+// @vitest-environment happy-dom
+
+import { act, createRef } from "react";
+import { createRoot } from "react-dom/client";
+import { describe, expect, it } from "vitest";
+import { useTimelinePlayhead } from "./useTimelinePlayhead";
+
+Object.assign(globalThis, { IS_REACT_ACT_ENVIRONMENT: true });
+
+function scrollBox(scrollLeft: number) {
+  const el = document.createElement("div");
+  let left = scrollLeft;
+  Object.defineProperties(el, {
+    clientWidth: { value: 800 },
+    scrollWidth: { value: 20_000 },
+    scrollLeft: { get: () => left, set: (v: number) => (left = v) },
+  });
+  return el;
+}
+
+function Harness({ pps, scroll }: { pps: number; scroll: HTMLDivElement }) {
+  useTimelinePlayhead({
+    playheadRef: createRef(),
+    scrollRef: { current: scroll },
+    ppsRef: { current: pps },
+    durationRef: { current: 60 },
+    isDragging: { current: false },
+    currentTime: 0,
+    zoomMode: "manual",
+    manualZoomPercent: 100,
+    zoomModeRef: { current: "manual" },
+    manualZoomPercentRef: { current: 100 },
+    fitPps: pps,
+    fitPpsRef: { current: pps },
+    effectiveDuration: 60,
+    pps,
+    timelineReady: true,
+    elementsLength: 1,
+    setZoomMode: () => {},
+    setManualZoomPercent: () => {},
+    contentOrigin: 32,
+  });
+  return null;
+}
+
+function zoom(scrollLeft: number, fromPps: number, toPps: number) {
+  const scroll = scrollBox(scrollLeft);
+  const host = document.createElement("div");
+  const root = createRoot(host);
+  act(() => root.render(<Harness pps={fromPps} scroll={scroll} />));
+  act(() => root.render(<Harness pps={toPps} scroll={scroll} />));
+  act(() => root.unmount());
+  return scroll.scrollLeft;
+}
+
+describe("useTimelinePlayhead centre anchor", () => {
+  it("keeps a view at the start at the start when the scale changes", () => {
+    expect(zoom(0, 100, 114)).toBe(0);
+  });
+
+  it("keeps the time at the viewport centre when the view is scrolled", () => {
+    // Centre time (400 + 400 - 32) / 100 = 7.68s lands at 32 + 7.68 * 200 - 400.
+    expect(zoom(400, 100, 200)).toBe(1168);
+  });
+});

packages/studio/src/player/components/useTimelinePlayhead.ts

@@ -75,7 +75,8 @@ export function useTimelinePlayhead({
     // it true and the next toolbar zoom would wrongly skip center-anchoring.
     const skip = skipCenterAnchorRef.current;
     skipCenterAnchorRef.current = false;
-    if (!scroll || pps === prevPps || skip) return;
+    // A view at the start stays there: a resize or a host zoom must not hide 00:00.
+    if (!scroll || pps === prevPps || skip || scroll.scrollLeft < 1) return;
     const nextScrollLeft = getTimelineScrollLeftForZoomAnchor({
       pointerX: scroll.clientWidth / 2,
       currentScrollLeft: scroll.scrollLeft,