msglist: Change how tests for ScrollToBottomButton tests for functionality

Switched from checking the ValueNotifier value to looking for the
existence of the tooltip in the visible button.

Author: sirpengi

test/widgets/message_list_test.dart

@@ -59,46 +59,50 @@ void main() {
       return stickyHeaderListView.controller;
     }
 
-    ScrollToBottomButton? findScrollToBottomButton(WidgetTester tester) {
-      return tester.widget<ScrollToBottomButton>(find.byType(ScrollToBottomButton));
+    bool isButtonVisible(WidgetTester tester) {
+      return tester.any(find.descendant(
+        of: find.byType(ScrollToBottomButton),
+        matching: find.byTooltip("Scroll to bottom")));
     }
 
     testWidgets('scrolling changes visibility', (WidgetTester tester) async {
       final stream = eg.stream();
       await setupMessageListPage(tester, narrow: StreamNarrow(stream.streamId));
 
       final scrollController = findMessageListScrollController(tester)!;
-      final scrollToBottomButton = findScrollToBottomButton(tester)!;
 
       // Initial state should be not visible, as the message list renders with latest message in view
-      check(scrollToBottomButton.visibleValue.value).equals(false);
+      check(isButtonVisible(tester)).equals(false);
 
       scrollController.jumpTo(600);
       await tester.pump();
-      check(scrollToBottomButton.visibleValue.value).equals(true);
+      check(isButtonVisible(tester)).equals(true);
 
       scrollController.jumpTo(0);
       await tester.pump();
-      check(scrollToBottomButton.visibleValue.value).equals(false);
+      check(isButtonVisible(tester)).equals(false);
     });
 
     testWidgets('dimension updates changes visibility', (WidgetTester tester) async {
       final stream = eg.stream();
       await setupMessageListPage(tester, narrow: StreamNarrow(stream.streamId));
 
       final scrollController = findMessageListScrollController(tester)!;
-      final scrollToBottomButton = findScrollToBottomButton(tester)!;
 
       // Initial state should be not visible, as the message list renders with latest message in view
-      check(scrollToBottomButton.visibleValue.value).equals(false);
+      check(isButtonVisible(tester)).equals(false);
 
       scrollController.jumpTo(600);
       await tester.pump();
-      check(scrollToBottomButton.visibleValue.value).equals(true);
+      check(isButtonVisible(tester)).equals(true);
 
       tester.view.physicalSize = const Size(2000, 40000);
       await tester.pump();
-      check(scrollToBottomButton.visibleValue.value).equals(false);
+      // Dimension changes use NotificationListener<ScrollMetricsNotification
+      // which has a one frame lag. If that ever gets resolved this extra pump
+      // would ideally be removed
+      await tester.pump();
+      check(isButtonVisible(tester)).equals(false);
     });
   });
-}
+}