test: stabilize transcript browser replay

Problem:
The Windows transcript browser gate intermittently sampled the dock resize before geometry settled, and its hydration setup could keep issuing wheel events while the viewport moved backward. Failures therefore changed signature across equivalent runs.

Root cause:
Two setup waits accepted a single transient frame, while the hydration driver treated only an unchanged position as stalled. Extent coverage failures also omitted the sampled geometry needed for classification.

Fix:
Require consecutive stable tail frames after dock resizes, end the reader lease whenever downward setup stops advancing, and include extent samples in failure diagnostics.

Verification:
- transcript-scroll-stability.mjs: 5 consecutive passes
- pnpm test:transcript-browser
- go run ./tools/repolint
- git diff --check

desktop/frontend/bench/transcript-scroll-stability.mjs

@@ -55,9 +55,9 @@ async function moveToOuterReaderGutter(page, transcript, announce = true) {
 
 async function waitForStableTranscriptGeometry(
   page,
-  { timeout = 15_000, frames = 8, requireTail = false } = {},
+  { timeout = 15_000, frames = 8, requireTail = false, maxTailDistance = 4 } = {},
 ) {
-  await page.evaluate(({ timeout, frames, requireTail }) => new Promise((resolve, reject) => {
+  await page.evaluate(({ timeout, frames, requireTail, maxTailDistance }) => new Promise((resolve, reject) => {
     const startedAt = performance.now();
     let previous = null;
     let stableFrames = 0;
@@ -80,7 +80,7 @@ async function waitForStableTranscriptGeometry(
         stableFrames = unchanged ? stableFrames + 1 : 0;
         previous = current;
         const distance = current.height - current.top - current.clientHeight;
-        const expectedPosition = !requireTail || (current.mode === "tail-follow" && distance <= 4);
+        const expectedPosition = !requireTail || (current.mode === "tail-follow" && distance <= maxTailDistance);
         if (expectedPosition && stableFrames >= frames) {
           resolve();
           return;
@@ -96,7 +96,7 @@ async function waitForStableTranscriptGeometry(
       requestAnimationFrame(sample);
     };
     requestAnimationFrame(sample);
-  }), { timeout, frames, requireTail });
+  }), { timeout, frames, requireTail, maxTailDistance });
 }
 
 async function openGeometryContractFixture(page) {
@@ -399,7 +399,7 @@ try {
     // A hydration resize intentionally guards the reader's previous extent
     // for one intent burst. Start a fresh burst after the 180ms idle lease
     // when that boundary is reached, matching a real user's next wheel turn.
-    const stalled = Math.abs(position.top - previousDownwardTop) <= 1;
+    const stalled = position.top <= previousDownwardTop + 1;
     previousDownwardTop = position.top;
     await page.waitForTimeout(stalled ? 220 : 16);
   }
@@ -623,10 +623,11 @@ try {
   // physical bottom. Keep the crop assertions tight; only the post-resize
   // stick-to-tail check gets this slack (CI saw 7px after collapse).
   const tailAfterResizePx = 16;
-  const waitNearTailAfterResize = () => page.waitForFunction((limit) => {
-    const scroller = document.querySelector(".transcript");
-    return Boolean(scroller && scroller.scrollHeight - scroller.scrollTop - scroller.clientHeight <= limit);
-  }, tailAfterResizePx);
+  const waitNearTailAfterResize = () => waitForStableTranscriptGeometry(page, {
+    frames: 4,
+    requireTail: true,
+    maxTailDistance: tailAfterResizePx,
+  });
 
   const collapse = page.getByRole("button", { name: /Collapse workspace|收起工作区/ });
   if (await collapse.count()) {
@@ -829,8 +830,11 @@ try {
     : `transient extent rebound uses one bounded reader-stability write (${readerStabilityWrites.length}; ${JSON.stringify(afterExtentReplay.samples)})`);
   assert(afterExtentReplay.writes.every((write) => write.owner !== "recovery"),
     "nonblank extent rebound never enters blank recovery");
-  assert(afterExtentReplay.samples.every((sample) => sample.occupied),
-    "transient extent rebound keeps mounted coverage in every sampled frame");
+  const hasContinuousExtentCoverage = afterExtentReplay.samples.every((sample) => sample.occupied);
+  assert(hasContinuousExtentCoverage,
+    hasContinuousExtentCoverage
+      ? "transient extent rebound keeps mounted coverage in every sampled frame"
+      : `transient extent rebound keeps mounted coverage in every sampled frame (${JSON.stringify(afterExtentReplay.samples)})`);
   await transcript.evaluate((element) => {
     delete element.dataset.extentReplayExpanded;
     document.getElementById("reader-extent-replay-style")?.remove();