dont mess with user supplied list (flutter#69594)

Author: LongCatIsLooong

packages/flutter/lib/src/cupertino/text_field.dart

@@ -878,13 +878,13 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
     super.build(context); // See AutomaticKeepAliveClientMixin.
     assert(debugCheckHasDirectionality(context));
     final TextEditingController controller = _effectiveController;
-    final List<TextInputFormatter> formatters = widget.inputFormatters ?? <TextInputFormatter>[];
     final TextSelectionControls textSelectionControls = widget.selectionControls ?? cupertinoTextSelectionControls;
     final bool enabled = widget.enabled ?? true;
     final Offset cursorOffset = Offset(_iOSHorizontalCursorOffsetPixels / MediaQuery.of(context).devicePixelRatio, 0);
-    if (widget.maxLength != null && widget.maxLengthEnforced) {
-      formatters.add(LengthLimitingTextInputFormatter(widget.maxLength));
-    }
+    final List<TextInputFormatter> formatters = <TextInputFormatter>[
+      ...?widget.inputFormatters,
+      if (widget.maxLength != null && widget.maxLengthEnforced) LengthLimitingTextInputFormatter(widget.maxLength)
+    ];
     final CupertinoThemeData themeData = CupertinoTheme.of(context);
 
     final TextStyle? resolvedStyle = widget.style?.copyWith(

packages/flutter/lib/src/material/text_field.dart

@@ -1261,9 +1261,10 @@ class _TextFieldState extends State<TextField> with RestorationMixin implements
     final Brightness keyboardAppearance = widget.keyboardAppearance ?? theme.primaryColorBrightness;
     final TextEditingController controller = _effectiveController;
     final FocusNode focusNode = _effectiveFocusNode;
-    final List<TextInputFormatter> formatters = widget.inputFormatters ?? <TextInputFormatter>[];
-    if (widget.maxLength != null && widget.maxLengthEnforced)
-      formatters.add(LengthLimitingTextInputFormatter(widget.maxLength));
+    final List<TextInputFormatter> formatters = <TextInputFormatter>[
+      ...?widget.inputFormatters,
+      if (widget.maxLength != null && widget.maxLengthEnforced) LengthLimitingTextInputFormatter(widget.maxLength)
+    ];
 
     TextSelectionControls? textSelectionControls = widget.selectionControls;
     final bool paintCursorAboveText;

packages/flutter/test/cupertino/text_field_test.dart

@@ -4289,4 +4289,16 @@ void main() {
     final EditableText widget = tester.widget(find.byType(EditableText));
     expect(widget.selectionControls, equals(selectionControl));
   });
+
+  testWidgets('Do not add LengthLimiting formatter to the user supplied list', (WidgetTester tester) async {
+    final List<TextInputFormatter> formatters = <TextInputFormatter>[];
+
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: CupertinoTextField(maxLength: 5, inputFormatters: formatters),
+      )
+    );
+
+    expect(formatters.isEmpty, isTrue);
+  });
 }

packages/flutter/test/material/text_field_test.dart

@@ -3344,6 +3344,22 @@ void main() {
     expect(textController.text, '145623');
   });
 
+  testWidgets('Do not add LengthLimiting formatter to the user supplied list', (WidgetTester tester) async {
+    final List<TextInputFormatter> formatters = <TextInputFormatter>[];
+
+    await tester.pumpWidget(
+      overlay(
+        child: TextField(
+          decoration: null,
+          maxLength: 5,
+          inputFormatters: formatters,
+        ),
+      ),
+    );
+
+    expect(formatters.isEmpty, isTrue);
+  });
+
   testWidgets('Text field scrolls the caret into view', (WidgetTester tester) async {
     final TextEditingController controller = TextEditingController();