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/hooks/useChatTimelineController.ts

@@ -60,6 +60,24 @@ export interface UseChatTimelineControllerResult {
 
 const TURN_MODEL_CACHE_MAX = 30
 const HISTORY_SCROLL_THRESHOLD = 200
+// On touch surfaces the user can drag continuously toward the top, and
+// loadEarlier is an async (network) fetch. A 200px lead is enough on desktop
+// (wheel + fast render) but the finger can outrun an in-flight fetch on mobile
+// and hit the very top before history lands. Give touch a much larger,
+// viewport-relative head start so the fetch completes before the top is
+// reached, regardless of how fast the user drags.
+const MOBILE_HISTORY_SCROLL_THRESHOLD_MIN = 1200
+const MOBILE_HISTORY_SCROLL_VIEWPORT_FACTOR = 2
+
+const resolveHistoryScrollThreshold = (clientHeight: number): number => {
+    if (!isMobileSurfaceRuntime()) {
+        return HISTORY_SCROLL_THRESHOLD
+    }
+    return Math.max(
+        MOBILE_HISTORY_SCROLL_THRESHOLD_MIN,
+        clientHeight * MOBILE_HISTORY_SCROLL_VIEWPORT_FACTOR,
+    )
+}
 const VSCODE_TURN_MODEL_CACHE_MAX = 4
 const VSCODE_TURN_MODEL_CACHE_MAX_MESSAGES = 30
 const MOBILE_TURN_MODEL_CACHE_MAX = 4
@@ -521,7 +539,7 @@ export const useChatTimelineController = ({
         const container = scrollRef.current;
         if (!container) return;
         if (isPinnedRef.current) return;
-        if (container.scrollTop >= HISTORY_SCROLL_THRESHOLD) return;
+        if (container.scrollTop >= resolveHistoryScrollThreshold(container.clientHeight)) return;
         if (!historySignalsRef.current.canLoadEarlier) return;
         if (isLoadingOlderRef.current || pendingRevealWorkRef.current) return;