fix(app): stop blank rows while scrolling tall Android chats (#7278)

Two causes, both reproduced on a clone of a real long thinking-partner chat
(replies and thinking rows several screens tall, GFM tables).

1. The window lowered in e42771970 (initial 2 / batch 2 / window 5) holds only
   two or three tall rows, so a fling outruns the mounted range and the viewport
   crosses unmounted rows: 65 blank frames over a 4,152-frame traversal. Keep the
   small initial render that bounds cold-open measurement, but refill a window of
   10 in batches of 10: 0 blank frames over 4,347, p95 25 ms.

2. Scroll-to-bottom from deep history animated across every row in between,
   which no finite window can mount in time: 9 blank frames (~150 ms) per jump.
   Animate only when the current offset is within the bottom-side overscan,
   otherwise jump instantly. Near-bottom jumps still animate. Web already does
   not animate this jump.

Cold open on the same chat is unchanged (first content 569 -> 568 ms); the
deeper window costs first-scroll acknowledgement (2.58 -> 2.94 s in a debug
build).

packages/app/src/agent-stream/native-scroll-animation.test.ts

@@ -0,0 +1,34 @@
+import { describe, expect, it } from "vitest";
+import { shouldAnimateNativeScrollToBottom } from "./native-scroll-animation";
+
+describe("native scroll-to-bottom animation", () => {
+  it("jumps across unmounted history instead of animating through blank rows", () => {
+    expect(
+      shouldAnimateNativeScrollToBottom({ offsetY: 60_000, viewportHeight: 600, windowSize: 10 }),
+    ).toBe(false);
+  });
+
+  it("keeps near-bottom movement eligible for animation", () => {
+    expect(
+      shouldAnimateNativeScrollToBottom({ offsetY: 300, viewportHeight: 600, windowSize: 10 }),
+    ).toBe(true);
+  });
+
+  it("bounds travel by the bottom-side overscan, not the whole window", () => {
+    expect(
+      shouldAnimateNativeScrollToBottom({ offsetY: 2700, viewportHeight: 600, windowSize: 10 }),
+    ).toBe(true);
+    expect(
+      shouldAnimateNativeScrollToBottom({ offsetY: 2701, viewportHeight: 600, windowSize: 10 }),
+    ).toBe(false);
+    expect(
+      shouldAnimateNativeScrollToBottom({ offsetY: 1500, viewportHeight: 600, windowSize: 5 }),
+    ).toBe(false);
+  });
+
+  it("does not animate before the viewport has been measured", () => {
+    expect(
+      shouldAnimateNativeScrollToBottom({ offsetY: 0, viewportHeight: 0, windowSize: 10 }),
+    ).toBe(false);
+  });
+});

packages/app/src/agent-stream/native-scroll-animation.ts

@@ -0,0 +1,10 @@
+export function shouldAnimateNativeScrollToBottom(input: {
+  offsetY: number;
+  viewportHeight: number;
+  windowSize: number;
+}): boolean {
+  // VirtualizedList splits the extra window around the viewport. An animated
+  // jump farther than the bottom-side overscan crosses unmounted history.
+  const bottomOverscanHeight = ((input.windowSize - 1) / 2) * input.viewportHeight;
+  return input.viewportHeight > 0 && input.offsetY <= bottomOverscanHeight;
+}

packages/app/src/agent-stream/strategy-native.tsx

@@ -23,6 +23,7 @@ import type { StreamItem } from "@/types/stream";
 import type { Theme } from "@/styles/theme";
 import { useStableEvent } from "@/hooks/use-stable-event";
 import { useKeyboardStreamInset } from "@/keyboard/shift";
+import { shouldAnimateNativeScrollToBottom } from "./native-scroll-animation";
 import { useRevisedHistoryRows } from "./history-row-revision";
 import { useBottomAnchorController } from "./bottom-anchor-controller";
 import { useScrollKeyboardDismiss } from "./scroll-keyboard-dismiss/use-scroll-keyboard-dismiss";
@@ -64,6 +65,7 @@ const historyStartSlotStyle: ViewStyle = {
   flexShrink: 0,
 };
 const HISTORY_START_SETTLE_FRAMES = 2;
+const NATIVE_STREAM_WINDOW_SIZE = 10;
 
 function keyExtractor(item: { id: string }): string {
   return item.id;
@@ -238,7 +240,13 @@ function NativeStreamViewport(props: StreamRenderInput & { strategy: StreamStrat
       programmaticScrollEventBudgetRef.current = 3;
       flatListRef.current?.scrollToOffset({
         offset: 0,
-        animated,
+        animated:
+          animated &&
+          shouldAnimateNativeScrollToBottom({
+            offsetY: scrollOffsetYRef.current,
+            viewportHeight: streamViewportMetricsRef.current.viewportHeight,
+            windowSize: NATIVE_STREAM_WINDOW_SIZE,
+          }),
       });
       scrollOffsetYRef.current = 0;
       streamViewportMetricsRef.current = {
@@ -591,11 +599,11 @@ function NativeStreamViewport(props: StreamRenderInput & { strategy: StreamStrat
       scrollEventThrottle={16}
       onContentSizeChange={handleContentSizeChange}
       maintainVisibleContentPosition={maintainVisibleContentPosition}
-      // A history row can contain an entire long reply. Keep synchronous native
-      // Markdown measurement bounded while FlatList fills the surrounding window.
+      // Tall Markdown rows exhaust a shallow window during flings. Keep the initial
+      // render small, then refill a deeper window in batches large enough to keep up.
       initialNumToRender={2}
-      maxToRenderPerBatch={2}
-      windowSize={5}
+      maxToRenderPerBatch={10}
+      windowSize={NATIVE_STREAM_WINDOW_SIZE}
       removeClippedSubviews={false}
       scrollEnabled={scrollEnabled}
       showsVerticalScrollIndicator