perf(timeline): eliminate room-open jitter, flash, and scroll drift

.changeset/fix-timeline-scroll-regressions.md

@@ -0,0 +1,5 @@
+---
+default: patch
+---
+
+Fix timeline scroll regressions: stay-at-bottom, Jump to Latest button flicker, phantom unread dot, and blank notification page recovery

.changeset/perf-timeline-item-memo.md

@@ -0,0 +1,5 @@
+---
+default: patch
+---
+
+Memoize individual VList timeline items to prevent mass re-renders when unrelated state changes (e.g. typing indicators, read receipts, or new messages while not at the bottom).

.changeset/perf-timeline-scroll-cache.md

@@ -0,0 +1,5 @@
+---
+default: patch
+---
+
+Cache VList item heights across room visits to restore scroll position instantly and skip the 80 ms opacity-fade stabilisation timer on revisit.

src/app/components/message/content/VideoContent.tsx

@@ -110,9 +110,12 @@ export const VideoContent = as<'div', VideoContentProps>(
       if (autoPlay) loadSrc();
     }, [autoPlay, loadSrc]);
 
+    const hasDimensions = typeof info?.w === 'number' && typeof info?.h === 'number';
+
     return (
       <Box
         className={classNames(css.RelativeBase, className)}
+        style={hasDimensions ? { aspectRatio: `${info.w} / ${info.h}` } : undefined}
         {...props}
         ref={ref}
         onPointerEnter={() => setIsHovered(true)}