gate conversation auto-scroll on the bottom pin (T-368)

New items arrive on every streamed token, and _onChanged jumped to
maxScrollExtent unconditionally — so a reader who scrolled up was
dragged back to the tail continuously for the whole reply. The
_atBottom pin already existed for viewport resizes (T-297); apply it
to the new-item path too, re-checking after layout since the user can
scroll during the frame. Twin tests added beside the T-297 pair:
pinned view keeps following, scrolled-up view stays put.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/builtin/claude/conversation_scroll_test.dart

@@ -1,6 +1,10 @@
 /// T-297: when the bottom interaction zone resizes, the conversation re-anchors
 /// to the tail (if pinned there) so content isn't left hidden behind the taller
 /// box — and leaves a scrolled-up reader undisturbed.
+///
+/// T-368: the same gate applies to NEW ITEMS — they arrive on every streamed
+/// token, and following the tail unconditionally yanked a scrolled-up reader
+/// to the bottom for the whole reply.
 library;
 
 import 'dart:async';
@@ -24,7 +28,7 @@ void main() {
 
   ScrollPosition scrollPos(WidgetTester tester) => tester.state<ScrollableState>(find.byType(Scrollable).first).position;
 
-  Future<ConversationController> pump(WidgetTester tester, ValueNotifier<double> bottomH) async {
+  Future<(ConversationController, StreamController<ConversationItem>)> pump(WidgetTester tester, ValueNotifier<double> bottomH) async {
     tester.view.physicalSize = const Size(600, 600);
     tester.view.devicePixelRatio = 1.0;
     addTearDown(() {
@@ -58,7 +62,7 @@ void main() {
       stream.add(_asst('conversation line number $i', i));
     }
     await tester.pumpAndSettle();
-    return c;
+    return (c, stream);
   }
 
   testWidgets('a growing bottom zone re-anchors the tail when pinned to bottom', (tester) async {
@@ -95,4 +99,42 @@ void main() {
     expect(after.pixels, closeTo(before, 1), reason: 'offset preserved; not re-anchored to bottom');
     expect(after.pixels, lessThan(after.maxScrollExtent - 8), reason: 'still not at the tail');
   });
+
+  testWidgets('new streamed items keep following the tail when pinned', (tester) async {
+    final bottomH = ValueNotifier<double>(40);
+    addTearDown(bottomH.dispose);
+    final (_, stream) = await pump(tester, bottomH);
+
+    final p = scrollPos(tester);
+    expect(p.pixels, closeTo(p.maxScrollExtent, 1), reason: 'starts pinned to the tail');
+
+    for (var i = 40; i < 60; i++) {
+      stream.add(_asst('streamed delta number $i', i));
+    }
+    await tester.pumpAndSettle();
+
+    final p2 = scrollPos(tester);
+    expect(p2.pixels, closeTo(p2.maxScrollExtent, 1), reason: 'still pinned after new items streamed in');
+  });
+
+  testWidgets('new streamed items do not yank a scrolled-up reader (T-368)', (tester) async {
+    final bottomH = ValueNotifier<double>(40);
+    addTearDown(bottomH.dispose);
+    final (_, stream) = await pump(tester, bottomH);
+
+    // Scroll up, away from the tail.
+    scrollPos(tester).jumpTo(30);
+    await tester.pump();
+    final before = scrollPos(tester).pixels;
+    expect(before, closeTo(30, 1));
+
+    for (var i = 40; i < 60; i++) {
+      stream.add(_asst('streamed delta number $i', i));
+    }
+    await tester.pumpAndSettle();
+
+    final after = scrollPos(tester);
+    expect(after.pixels, closeTo(before, 1), reason: 'reading position preserved while the reply streams');
+    expect(after.pixels, lessThan(after.maxScrollExtent - 8), reason: 'still not at the tail');
+  });
 }