msglist: Add test for ScrollToBottomButton functionality when pressed

Author: sirpengi

test/widgets/message_list_test.dart

@@ -104,5 +104,23 @@ void main() {
       await tester.pump();
       check(isButtonVisible(tester)).equals(false);
     });
+
+    testWidgets('button functionality', (WidgetTester tester) async {
+      final stream = eg.stream();
+      await setupMessageListPage(tester, narrow: StreamNarrow(stream.streamId));
+
+      final scrollController = findMessageListScrollController(tester)!;
+
+      // Initial state should be not visible, as the message list renders with latest message in view
+      check(isButtonVisible(tester)).equals(false);
+
+      scrollController.jumpTo(600);
+      await tester.pump();
+      check(isButtonVisible(tester)).equals(true);
+
+      await tester.tap(find.byType(ScrollToBottomButton));
+      await tester.pumpAndSettle();
+      check(isButtonVisible(tester)).equals(false);
+    });
   });
 }