Fix cursor disappear on undo. (#122402)

Fix cursor disappear on undo.

packages/flutter/lib/src/widgets/editable_text.dart

@@ -4510,7 +4510,7 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
                 userUpdateTextEditingValue(value, SelectionChangedCause.keyboard);
               },
               shouldChangeUndoStack: (TextEditingValue? oldValue, TextEditingValue newValue) {
-                if (newValue == TextEditingValue.empty) {
+                if (!newValue.selection.isValid) {
                   return false;
                 }
 

packages/flutter/lib/src/widgets/undo_history.dart

@@ -100,7 +100,19 @@ class UndoHistoryState<T> extends State<UndoHistory<T>> with UndoManagerClient {
 
   @override
   void undo() {
+    if (_stack.currentValue == null)  {
+      // Returns early if there is not a first value registered in the history.
+      // This is important because, if an undo is received while the initial
+      // value is being pushed (a.k.a when the field gets the focus but the
+      // throttling delay is pending), the initial push should not be canceled.
+      return;
+    }
+    if (_throttleTimer?.isActive ?? false) {
+      _throttleTimer?.cancel(); // Cancel ongoing push, if any.
+      _update(_stack.currentValue);
+    } else {
       _update(_stack.undo());
+    }
     _updateState();
   }
 
@@ -455,27 +467,17 @@ typedef _Throttled<T> = Timer Function(T currentArg);
 _Throttled<T> _throttle<T>({
   required Duration duration,
   required _Throttleable<T> function,
-  // If true, calls at the start of the timer.
-  bool leadingEdge = false,
 }) {
   Timer? timer;
-  bool calledDuringTimer = false;
   late T arg;
 
   return (T currentArg) {
     arg = currentArg;
-    if (timer != null) {
-      calledDuringTimer = true;
+    if (timer != null && timer!.isActive) {
       return timer!;
     }
-    if (leadingEdge) {
-      function(arg);
-    }
-    calledDuringTimer = false;
     timer = Timer(duration, () {
-      if (!leadingEdge || calledDuringTimer) {
       function(arg);
-      }
       timer = null;
     });
     return timer!;

packages/flutter/test/widgets/editable_text_test.dart

@@ -13011,6 +13011,58 @@ testWidgets('Floating cursor ending with selection', (WidgetTester tester) async
     // On web, these keyboard shortcuts are handled by the browser.
     }, variant: TargetPlatformVariant.only(TargetPlatform.android), skip: kIsWeb); // [intended]
 
+    // Regression test for https://github.com/flutter/flutter/issues/120194.
+    testWidgets('Cursor does not jump after undo', (WidgetTester tester) async {
+      // Initialize the controller with a non empty text.
+      final TextEditingController controller = TextEditingController(text: textA);
+      final FocusNode focusNode = FocusNode();
+      await tester.pumpWidget(boilerplate(controller, focusNode));
+
+      // Focus the field and wait for throttling delay to get the initial
+      // state saved in text editing history.
+      focusNode.requestFocus();
+      await tester.pump();
+      await waitForThrottling(tester);
+      expect(controller.value, textACollapsedAtEnd);
+
+      // Insert some text.
+      await tester.enterText(find.byType(EditableText), textAB);
+      expect(controller.value, textABCollapsedAtEnd);
+
+      // Undo the insertion without waiting for the throttling delay.
+      await sendUndo(tester);
+      expect(controller.value.selection.isValid, true);
+      expect(controller.value, textACollapsedAtEnd);
+
+    // On web, these keyboard shortcuts are handled by the browser.
+    }, variant: TargetPlatformVariant.all(), skip: kIsWeb); // [intended]
+
+    testWidgets('Initial value is recorded when an undo is received just after getting the focus', (WidgetTester tester) async {
+      // Initialize the controller with a non empty text.
+      final TextEditingController controller = TextEditingController(text: textA);
+      final FocusNode focusNode = FocusNode();
+      await tester.pumpWidget(boilerplate(controller, focusNode));
+
+      // Focus the field and do not wait for throttling delay before calling undo.
+      focusNode.requestFocus();
+      await tester.pump();
+      await sendUndo(tester);
+      await waitForThrottling(tester);
+      expect(controller.value, textACollapsedAtEnd);
+
+      // Insert some text.
+      await tester.enterText(find.byType(EditableText), textAB);
+      expect(controller.value, textABCollapsedAtEnd);
+
+      // Undo the insertion.
+      await sendUndo(tester);
+
+      // Initial text should have been recorded and restored.
+      expect(controller.value, textACollapsedAtEnd);
+
+    // On web, these keyboard shortcuts are handled by the browser.
+    }, variant: TargetPlatformVariant.all(), skip: kIsWeb); // [intended]
+
     testWidgets('Can make changes in the middle of the history', (WidgetTester tester) async {
       final TextEditingController controller = TextEditingController();
       final FocusNode focusNode = FocusNode();