Length formatter minor fix (#66567)

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

@@ -319,8 +319,8 @@ class LengthLimitingTextInputFormatter extends TextInputFormatter {
   /// The limit on the number of characters (i.e. Unicode scalar values) this formatter
   /// will allow.
   ///
-  /// The value must be null or greater than zero. If it is null, then no limit
-  /// is enforced.
+  /// The value must be null or greater than zero. If it is null or -1, then no
+  /// limit is enforced.
   ///
   /// This formatter does not currently count Unicode grapheme clusters (i.e.
   /// characters visible to the user), it counts Unicode scalar values, which leaves
@@ -379,25 +379,22 @@ class LengthLimitingTextInputFormatter extends TextInputFormatter {
     TextEditingValue oldValue,
     TextEditingValue newValue,
   ) {
-    // Return the new value when the old value has not reached the max
-    // limit or the old value is composing too.
-    if (newValue.composing.isValid) {
-      if (maxLength != null && maxLength! > 0 &&
-          oldValue.text.characters.length == maxLength! &&
-          !oldValue.composing.isValid) {
-        return oldValue;
-      }
+    final int? maxLength = this.maxLength;
+
+    if (maxLength == null || maxLength == -1 || newValue.text.characters.length <= maxLength)
       return newValue;
-    }
-    if (maxLength != null && maxLength! > 0 && newValue.text.characters.length > maxLength!) {
+
+    assert(maxLength > 0);
+
     // If already at the maximum and tried to enter even more, keep the old
     // value.
-      if (oldValue.text.characters.length == maxLength) {
+    if (oldValue.text.characters.length == maxLength && !oldValue.composing.isValid) {
       return oldValue;
     }
-      return truncate(newValue, maxLength!);
-    }
-    return newValue;
+
+    // Temporarily exempt `newValue` from the maxLength limit if it has a
+    // composing text going, until the composing is finished.
+    return newValue.composing.isValid ? newValue : truncate(newValue, maxLength);
   }
 }
 

packages/flutter/test/widgets/editable_text_test.dart

@@ -5734,6 +5734,43 @@ void main() {
     expect(state.currentTextEditingValue.text, '12345');
     expect(state.currentTextEditingValue.composing, TextRange.empty);
   });
+
+  testWidgets('Length formatter handles composing text correctly, continued', (WidgetTester tester) async {
+    final TextInputFormatter formatter = LengthLimitingTextInputFormatter(5);
+    final Widget widget = MaterialApp(
+      home: EditableText(
+        backgroundCursorColor: Colors.grey,
+        controller: controller,
+        focusNode: focusNode,
+        inputFormatters: <TextInputFormatter>[formatter],
+        style: textStyle,
+        cursorColor: cursorColor,
+        selectionControls: materialTextSelectionControls,
+      ),
+    );
+
+    await tester.pumpWidget(widget);
+    final EditableTextState state = tester.state<EditableTextState>(find.byType(EditableText));
+
+    // Initially we're at maxLength with no composing text.
+    controller.text = '12345' ;
+    assert(state.currentTextEditingValue == const TextEditingValue(text: '12345'));
+
+    // Should be able to change the editing value if the new value is still shorter
+    // than maxLength.
+    state.updateEditingValue(const TextEditingValue(text: '12345', composing: TextRange(start: 2, end: 4)));
+    expect(state.currentTextEditingValue.composing, const TextRange(start: 2, end: 4));
+
+    // Reset.
+    controller.text = '12345' ;
+    assert(state.currentTextEditingValue == const TextEditingValue(text: '12345'));
+
+    // The text should not change when trying to insert when the text is already
+    // at maxLength.
+    state.updateEditingValue(const TextEditingValue(text: 'abcdef', composing: TextRange(start: 5, end: 6)));
+    expect(state.currentTextEditingValue.text, '12345');
+    expect(state.currentTextEditingValue.composing, TextRange.empty);
+  });
 }
 
 class MockTextFormatter extends TextInputFormatter {