Merge main (anchored-turn chat scrolling) into perf/switch-and-scroll

Main replaced the chat timeline scroll engine while this branch was in
flight, which obsoletes two of its subareas and reshapes a third:

- Chat timeline: main's LegendList-based MessageList/ChatContainer win;
  the activation-overscan staircase targeted the removed tanstack path
  (LegendList provides adaptive rendering natively) and is dropped along
  with its test.
- Scroll shadows: main's hook-based masks stay (the virtualized list owns
  its scroll element — there is no wrapper to hand the styling to); the
  viewport-wrapper ScrollShadow rewrite, its index.css replacement, its
  test, and the call-site viewportClassName adaptations are reverted to
  main. The chat OverlayScrollbar keeps this branch's disableHorizontal.
- OverlayScrollbar: the direct-DOM rewrite lands, but binding now follows
  the live container node instead of binding once per ref object — the
  chat scroller remounts on every session switch, and a bind-once
  contract left the scrollbar attached to a dead element.
- Markdown renderer: the detached-DOM cache and warm-block fast path
  merge with main's block-commit reveal (enter cascade), streaming code
  highlighting, and gutter reservation; the per-block reconcile keeps
  both the decoration-refresh path and the reveal cascade.

packages/ui/src/components/chat/markdown/decorate.ts

@@ -133,6 +133,9 @@ const layoutCodeLines = (pre: HTMLPreElement): void => {
   const code = pre.querySelector<HTMLElement>(':scope > code');
   if (!code || code.hasAttribute('data-md-code-lines')) return;
 
+  // The real gutter takes over the reserved footprint.
+  pre.removeAttribute('data-md-gutter-reserved');
+
   const text = code.textContent ?? '';
   const hasTrailingNewline = text.endsWith('\n');
   const lines = hasTrailingNewline ? text.slice(0, -1).split('\n') : text.split('\n');
@@ -265,7 +268,15 @@ const decorateCodeBlocks = (root: HTMLElement, ctx: DecorateContext): void => {
     pre.style.margin = '0';
     pre.style.background = 'transparent';
     pre.classList.add('min-w-0', 'w-full', 'flex-1');
-    if (!ctx.deferCodeLineNumberSync) layoutCodeLines(pre);
+    if (!ctx.deferCodeLineNumberSync) {
+      layoutCodeLines(pre);
+    } else {
+      // Streaming defers the per-line gutter markup, but the gutter's
+      // horizontal footprint is reserved immediately — otherwise the
+      // end-of-stream decorate pass shifts every code line right by the
+      // gutter column and the finished message visibly jumps.
+      pre.setAttribute('data-md-gutter-reserved', '');
+    }
     body.appendChild(pre);
     wrapper.appendChild(header);
     wrapper.appendChild(body);

packages/ui/src/components/chat/markdown/markdownCore.ts

@@ -178,9 +178,11 @@ type MarkdownBlock = {
   raw: string;
   src: string;
   mode: 'full' | 'live';
-  // When false, skip syntax highlighting for this block. Set for the actively
-  // streaming open code fence so we don't re-tokenize a growing block ~40x/sec
-  // (O(n^2)); it highlights once the fence closes and becomes a stable block.
+  // When false, skip syntax highlighting for this block. Block-level commit
+  // feeds the open fence whole lines at the throttle cadence (<=10/sec), so a
+  // partial fence highlights too and streamed code arrives colored; only a
+  // very large open fence falls back to plain text until it closes, keeping
+  // the repeated worker re-tokenization bounded.
   highlight: boolean;
 };
 
@@ -201,6 +203,11 @@ const hasOpenFence = (raw: string): boolean => {
   return !new RegExp(`^[\\t ]{0,3}${char}{${size},}[\\t ]*$`).test(last);
 };
 
+// Above this, re-highlighting the still-open fence on every committed line
+// costs more than the colored preview is worth; the block highlights in one
+// pass when the fence closes.
+const OPEN_FENCE_HIGHLIGHT_LINE_LIMIT = 300;
+
 const heal = (text: string): string => {
   try {
     return remend(text, { linkMode: 'text-only' });
@@ -250,11 +257,13 @@ const streamBlocks = (text: string, live: boolean): MarkdownBlock[] => {
     const raw = token.raw ?? '';
     const isLast = i === tail;
     const openFence = token.type === 'code' && hasOpenFence(raw);
+    const openFenceHighlight = openFence
+      && raw.split('\n').length <= OPEN_FENCE_HIGHLIGHT_LINE_LIMIT;
     blocks.push({
       raw,
       src: openFence ? raw : heal(raw),
       mode: isLast ? 'live' : 'full',
-      highlight: !openFence,
+      highlight: !openFence || openFenceHighlight,
     });
   }