[keyboard_textfield_test] wait until the keyboard becomes visible (#92032)

dev/integration_tests/ui/lib/keyboard_textfield.dart

@@ -47,14 +47,24 @@ class _MyHomePageState extends State<MyHomePage> {
     });
   }
 
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
   @override
   Widget build(BuildContext context) {
+    final String softKeyboardVisibility = MediaQuery.of(context).viewInsets.bottom > 100
+      ? 'keyboard visible'
+      : 'keyboard not visible';
     return Scaffold(
       body: Column(
         children: <Widget>[
           Text('$offset',
             key: const ValueKey<String>(keys.kOffsetText),
           ),
+          Text(softKeyboardVisibility),
           Expanded(
             child: ListView(
               key: const ValueKey<String>(keys.kListView),

dev/integration_tests/ui/test_driver/keyboard_textfield_test.dart

@@ -24,6 +24,7 @@ void main() {
       final SerializableFinder listViewFinder = find.byValueKey(keys.kListView);
       final SerializableFinder textFieldFinder = find.byValueKey(keys.kDefaultTextField);
       final SerializableFinder offsetFinder = find.byValueKey(keys.kOffsetText);
+      final SerializableFinder keyboardVisibilityIndicatorFinder = find.text('keyboard visible');
 
       // Align TextField with bottom edge to ensure it would be covered when keyboard comes up.
       await driver.waitForAbsent(textFieldFinder);
@@ -38,7 +39,7 @@ void main() {
 
       // Bring up keyboard
       await driver.tap(textFieldFinder);
-      await Future<void>.delayed(const Duration(seconds: 1));
+      await driver.waitFor(keyboardVisibilityIndicatorFinder);
 
       // Ensure that TextField is visible again
       await driver.waitFor(textFieldFinder);