fix(chat): sticky user headers float mid-list in the virtualized timeline

The tanstack rows sit in a wrapper offset with transform: translateY(), and a
transformed ancestor becomes the sticky containing block — turn headers stuck
to the wrapper's overscan-dependent top edge instead of the scroll container,
floating over the previous turn. Offset the wrapper with padding-top instead:
identical geometry, sticky computes against the scroll container again, and
the padding only changes when the virtual window shifts, not per scroll frame.

packages/ui/src/components/chat/MessageList.tsx

@@ -1193,12 +1193,17 @@ const StaticHistoryList = React.memo(({ entries, engine, contentRef, scrollRef,
     if (engine === 'tanstack') {
         const virtualItems = tanstackVirtualizer.getVirtualItems();
         const startOffset = virtualItems[0]?.start ?? 0;
-        // Rendered rows stay in normal flow inside a single translated wrapper
-        // (not per-row absolute positioning) so per-turn sticky user headers
-        // keep working against the scroll container.
+        // Rendered rows stay in normal flow inside a single offset wrapper (not
+        // per-row absolute positioning) so per-turn sticky user headers keep
+        // working against the scroll container. The offset MUST be padding, not
+        // transform: a transformed ancestor becomes the sticky containing block,
+        // so headers would stick to the wrapper's (arbitrary, overscan-dependent)
+        // top edge mid-list and float over the previous turn. Padding only
+        // changes when the virtual window shifts — not per scroll frame — so the
+        // layout cost is negligible.
         return (
             <div ref={sizeContainerRef} className="relative w-full" style={{ height: tanstackVirtualizer.getTotalSize() }}>
-                <div style={{ transform: `translateY(${startOffset}px)` }}>
+                <div style={{ paddingTop: `${startOffset}px` }}>
                     {virtualItems.map((item) => {
                         const entry = renderEntries[item.index];
                         if (!entry) return null;