fix(chat): improve mobile history loading and virtualization fidelity

Give touch surfaces a larger, viewport-relative head start for loading older
history so an in-flight fetch completes before the finger reaches the top.
Raise the mobile virtualizer overscan so fast flings stay populated instead of
leaving blank gaps, and drop the fixed itemSize hint so virtua auto-estimates
row heights from measured sizes instead of a flat constant.

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

@@ -19,25 +19,22 @@ import type { StreamPhase } from './message/types';
 import { useGlobalSessionsStore } from '@/stores/useGlobalSessionsStore';
 import { useSessionParts } from '@/sync/sync-context';
 import type { ReviewTransferDirection } from '@/lib/reviewFlow';
+import { isMobileSurfaceRuntime } from '@/lib/runtimeSurface';
 
 const MESSAGE_LIST_VIRTUALIZE_THRESHOLD = 5;
 const EMPTY_STATIC_ENTRY_MESSAGES: ChatMessageEntry[] = [];
 const EMPTY_UNGROUPED_MESSAGE_IDS = new Set<string>();
 const MESSAGE_LIST_BUFFER_SIZE = 900;
+// Touch surfaces fling-scroll natively and dispatch scroll events less often
+// than the virtualizer can repaint, so a desktop-sized buffer leaves blank gaps
+// during momentum that only fill once measurement catches up. A larger overscan
+// keeps more rows mounted around the viewport so fast flings stay populated.
+const MOBILE_MESSAGE_LIST_BUFFER_SIZE = 2400;
+const resolveMessageListBufferSize = (): number => (
+    isMobileSurfaceRuntime() ? MOBILE_MESSAGE_LIST_BUFFER_SIZE : MESSAGE_LIST_BUFFER_SIZE
+);
 const TIMELINE_CACHE_LIMIT = 16;
 
-const estimateHistoryEntryHeight = (entry: RenderEntry | undefined): number => {
-    if (!entry) {
-        return 160;
-    }
-
-    if (entry.kind === 'turn') {
-        return 180 + Math.min(entry.turn.assistantMessages.length, 4) * 100;
-    }
-
-    return 140;
-};
-
 const sameKeys = (a: readonly string[] | undefined, b: readonly string[] | undefined): boolean => {
     if (a === b) return true;
     if (!a || !b) return false;
@@ -982,8 +979,7 @@ const StaticHistoryList = React.memo(({ entries, shouldVirtualize, contentRef, s
             ref={virtualizerRef}
             data={entries}
             cache={virtualCache}
-            itemSize={virtualCache ? undefined : estimateHistoryEntryHeight(undefined)}
-            bufferSize={MESSAGE_LIST_BUFFER_SIZE}
+            bufferSize={resolveMessageListBufferSize()}
             shift={shift}
             scrollRef={scrollRef}
         >