fix(frontend): wait for mounted extent rebound

Problem: WebView2 can restore the native transcript extent before Virtuoso has remounted visible rows, so reader-stability correction can expose a blank frame.

Root cause: the correction was eligible on the first rebound sample and did not require mounted viewport coverage or stable restored geometry.

Fix: gate the single correction on a nonblank viewport and one unchanged-height interval, with a regression test covering the rebound race.

Verification: pnpm test:transcript; pnpm test:transcript-browser; five consecutive transcript scroll stability replays; pnpm test:remote; pnpm test:performance; pnpm build; repolint.

desktop/frontend/src/__tests__/transcript-reader-extent-race.test.tsx

@@ -69,8 +69,12 @@ const rectAt = (top: number) => ({
 });
 const scrollElement = dom.window.document.getElementById("scroll") as HTMLDivElement;
 const rowElement = scrollElement.querySelector<HTMLElement>(".transcript__row")!;
+const reboundCoverageRow = dom.window.document.createElement("div");
+reboundCoverageRow.className = "transcript__row";
+reboundCoverageRow.dataset.rowKey = "row-ready";
 scrollElement.getBoundingClientRect = () => rectAt(0);
 rowElement.getBoundingClientRect = () => rectAt(20);
+reboundCoverageRow.getBoundingClientRect = () => rectAt(20);
 Object.defineProperty(scrollElement, "clientHeight", { configurable: true, value: 725 });
 let scrollExtent = 15_829;
 Object.defineProperty(scrollElement, "scrollHeight", { configurable: true, get: () => scrollExtent });
@@ -145,13 +149,18 @@ await act(async () => arbiter?.followGrowingTail());
 await flushFrames();
 check(scrollByCalls === 0, "the transaction waits while the native extent remains collapsed");
 scrollExtent = 15_829;
+scrollElement.append(reboundCoverageRow);
 await act(async () => arbiter?.followGrowingTail());
 await flushFrames();
+check(scrollByCalls === 0,
+  "the rebound waits for mounted coverage and stable restored geometry");
+await flushFrames();
 check(scrollByCalls === 1 && lastScrollByTop > 1_900,
   `the rebound restores the logical anchor exactly once (${lastScrollByTop}px)`);
 check(scrollWrites.length === 1 && scrollWrites[0].owner === "reader-stability",
   "the correction is owned by reader stability rather than recovery or tail-follow");
 check(arbiter?.modeRef.current === "manual", "the correction preserves manual reader ownership");
+reboundCoverageRow.remove();
 
 // Touch movement is incremental: the second touchmove protects only its own
 // segment rather than replaying the distance from the original touchstart.
@@ -173,13 +182,16 @@ await act(async () => arbiter?.onTouchMoveIntent({
 scrollExtent = 4_000;
 scrollElement.scrollTop = 1_000;
 rowElement.remove();
+scrollElement.append(reboundCoverageRow);
 await act(async () => arbiter?.deliverScroll());
 scrollExtent = 5_000;
 scrollByCalls = 0;
 lastScrollByTop = 0;
 await flushFrames();
+await flushFrames();
 check(scrollByCalls === 1 && lastScrollByTop === 1_020,
   `consecutive touch segments use incremental geometry (${lastScrollByTop}px)`);
+reboundCoverageRow.remove();
 scrollElement.append(rowElement);
 
 // Ordinary sub-viewport measurement jitter stays browser-owned, and a higher

desktop/frontend/src/lib/useTranscriptReaderExtentStability.ts

@@ -10,7 +10,10 @@ import {
 import type { TranscriptScrollMode } from "./transcriptScrollArbiter";
 import { nativeTranscriptDistanceFromBottom } from "./transcriptScrollGeometry";
 import type { TranscriptScrollWriteRecord } from "./transcriptScrollProbe";
-import { captureTranscriptLayoutAnchor } from "./transcriptVirtuosoRecovery";
+import {
+  captureTranscriptLayoutAnchor,
+  transcriptElementViewportIsBlank,
+} from "./transcriptVirtuosoRecovery";
 
 const READER_EXTENT_STABILITY_MS = 180;
 
@@ -68,6 +71,7 @@ export function useTranscriptReaderExtentStability({
       deadline: Date.now() + READER_EXTENT_STABILITY_MS,
       frame: null,
     };
+    let reboundHeight: number | undefined;
     const tick = () => {
       active.frame = null;
       if (
@@ -85,7 +89,15 @@ export function useTranscriptReaderExtentStability({
         clientHeight: element.clientHeight,
       };
       observeTranscriptReaderExtent(active, snapshot);
-      if (transcriptReaderExtentCanCorrect(active, snapshot)) {
+      // WebView2 can publish the restored native extent one or two paints
+      // before Virtuoso mounts rows for it. Writing during that gap moves the
+      // viewport into an empty size tree. Wait for mounted coverage and one
+      // unchanged-height interval, then spend the single correction budget.
+      if (
+        transcriptReaderExtentCanCorrect(active, snapshot)
+        && !transcriptElementViewportIsBlank(element)
+        && reboundHeight === snapshot.scrollHeight
+      ) {
         const row = active.anchor
           ? Array.from(element.querySelectorAll<HTMLElement>(".transcript__row[data-row-key]"))
             .find((candidate) => candidate.dataset.rowKey === active.anchor?.rowKey)
@@ -109,6 +121,7 @@ export function useTranscriptReaderExtentStability({
           return;
         }
       }
+      reboundHeight = snapshot.scrollHeight;
       if (Date.now() >= active.deadline) {
         guardRef.current = null;
         return;