fix length formatter typo (#79489)

packages/flutter/lib/src/services/text_formatter.dart

@@ -502,7 +502,7 @@ class LengthLimitingTextInputFormatter extends TextInputFormatter {
       case MaxLengthEnforcement.enforced:
         // If already at the maximum and tried to enter even more, and has no
         // selection, keep the old value.
-        if (oldValue.text.characters.length == maxLength && !oldValue.selection.isValid) {
+        if (oldValue.text.characters.length == maxLength && oldValue.selection.isCollapsed) {
           return oldValue;
         }
 

packages/flutter/test/material/text_field_test.dart

@@ -3864,6 +3864,90 @@ void main() {
     expect(textController.text, testValue);
   });
 
+  testWidgets(
+    'maxLength limits input in the center of a maxed-out field, with collapsed selection',
+    (WidgetTester tester) async {
+      final TextEditingController textController = TextEditingController();
+      const String testValue = '0123456789';
+
+      await tester.pumpWidget(boilerplate(
+        child: TextField(
+          controller: textController,
+          maxLength: 10,
+        ),
+      ));
+
+      // Max out the character limit in the field.
+      await tester.showKeyboard(find.byType(TextField));
+      tester.testTextInput.updateEditingValue(const TextEditingValue(
+        text: testValue,
+        selection: TextSelection.collapsed(offset: 10),
+        composing: TextRange.empty,
+      ));
+      await tester.pump();
+      expect(textController.text, testValue);
+
+      // Entering more characters at the end does nothing.
+      await tester.showKeyboard(find.byType(TextField));
+      tester.testTextInput.updateEditingValue(const TextEditingValue(
+        text: testValue + '9999999',
+        selection: TextSelection.collapsed(offset: 10 + 7),
+        composing: TextRange.empty,
+      ));
+      await tester.pump();
+
+      expect(textController.text, testValue);
+
+      // Entering text in the middle of the field also does nothing.
+      // Entering more characters at the end does nothing.
+      await tester.showKeyboard(find.byType(TextField));
+      tester.testTextInput.updateEditingValue(const TextEditingValue(
+        text: '0123455555555556789',
+        selection: TextSelection.collapsed(offset: 19),
+        composing: TextRange.empty,
+      ));
+      await tester.pump();
+
+      expect(textController.text, testValue);
+    },
+  );
+
+  testWidgets(
+    'maxLength limits input in the center of a maxed-out field, with non-collapsed selection',
+    (WidgetTester tester) async {
+      final TextEditingController textController = TextEditingController();
+      const String testValue = '0123456789';
+
+      await tester.pumpWidget(boilerplate(
+        child: TextField(
+          controller: textController,
+          maxLength: 10,
+        ),
+      ));
+
+      // Max out the character limit in the field.
+      await tester.showKeyboard(find.byType(TextField));
+      tester.testTextInput.updateEditingValue(const TextEditingValue(
+        text: testValue,
+        selection: TextSelection(baseOffset: 8, extentOffset: 10),
+        composing: TextRange.empty,
+      ));
+      await tester.pump();
+      expect(textController.text, testValue);
+
+      // Entering more characters at the end does nothing.
+      await tester.showKeyboard(find.byType(TextField));
+      tester.testTextInput.updateEditingValue(const TextEditingValue(
+        text: '01234569999999',
+        selection: TextSelection.collapsed(offset: 14),
+        composing: TextRange.empty,
+      ));
+      await tester.pump();
+
+      expect(textController.text,  '0123456999');
+    },
+  );
+
   testWidgets('maxLength limits input length even if decoration is null.', (WidgetTester tester) async {
     final TextEditingController textController = TextEditingController();