Fix `DropdownButtonFormField` input border clipping (#131481)

fixes [BUG: DropdownButtonFormField input decorator focus/hover is not clipped
](https://github.com/flutter/flutter/issues/131282)

### Description

This fixes an issue where `DropdownButtonFormField`'s input border isn't used for clipping with `InkWell`.

### Before
![Screenshot 2023-07-28 at 17 06 20](https://github.com/flutter/flutter/assets/48603081/8fe1ee1f-5cea-4297-b4f6-e672d74bb583)

### After
![Screenshot 2023-07-28 at 17 06 38](https://github.com/flutter/flutter/assets/48603081/a0091459-67dc-45ca-96b1-95d7093d475f)

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

@@ -15,6 +15,7 @@ import 'constants.dart';
 import 'debug.dart';
 import 'icons.dart';
 import 'ink_well.dart';
+import 'input_border.dart';
 import 'input_decorator.dart';
 import 'material.dart';
 import 'material_localizations.dart';
@@ -1633,6 +1634,7 @@ class DropdownButtonFormField<T> extends FormField<T> {
              }
            }
            final bool isEmpty = !showSelectedItem && !isHintOrDisabledHintAvailable();
+           final bool hasError = effectiveDecoration.errorText != null;
 
            // An unfocusable Focus widget so that this widget can detect if its
            // descendants have focus or not.
@@ -1640,6 +1642,33 @@ class DropdownButtonFormField<T> extends FormField<T> {
              canRequestFocus: false,
              skipTraversal: true,
              child: Builder(builder: (BuildContext context) {
+                final bool isFocused = Focus.of(context).hasFocus;
+                InputBorder? resolveInputBorder() {
+                  if (hasError) {
+                    if (isFocused) {
+                      return effectiveDecoration.focusedErrorBorder;
+                    }
+                    return effectiveDecoration.errorBorder;
+                  }
+                  if (isFocused) {
+                    return effectiveDecoration.focusedBorder;
+                  }
+                  if (effectiveDecoration.enabled) {
+                    return effectiveDecoration.enabledBorder;
+                  }
+                  return effectiveDecoration.border;
+                }
+                BorderRadius? effectiveBorderRadius() {
+                  final InputBorder? inputBorder = resolveInputBorder();
+                  if (inputBorder is OutlineInputBorder) {
+                    return inputBorder.borderRadius;
+                  }
+                  if (inputBorder is UnderlineInputBorder) {
+                    return inputBorder.borderRadius;
+                  }
+                  return null;
+                }
+
                return DropdownButtonHideUnderline(
                  child: DropdownButton<T>._formField(
                    items: items,
@@ -1665,10 +1694,10 @@ class DropdownButtonFormField<T> extends FormField<T> {
                    menuMaxHeight: menuMaxHeight,
                    enableFeedback: enableFeedback,
                    alignment: alignment,
-                   borderRadius: borderRadius,
+                   borderRadius: borderRadius ?? effectiveBorderRadius(),
                    inputDecoration: effectiveDecoration.copyWith(errorText: field.errorText),
                    isEmpty: isEmpty,
-                   isFocused: Focus.of(context).hasFocus,
+                   isFocused: isFocused,
                    padding: padding,
                  ),
                );

packages/flutter/test/material/dropdown_form_field_test.dart

@@ -4,6 +4,7 @@
 
 import 'dart:math' as math;
 
+import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -1134,4 +1135,100 @@ void main() {
       selectedItemBox.localToGlobal(Offset(selectedItemBox.size.width / 2.0, selectedItemBox.size.height / 2.0)),
     );
   });
+
+  testWidgets('InputDecoration borders are used for clipping', (WidgetTester tester) async {
+    const BorderRadius errorBorderRadius = BorderRadius.all(Radius.circular(5.0));
+    const BorderRadius focusedErrorBorderRadius = BorderRadius.all(Radius.circular(6.0));
+    const BorderRadius focusedBorder = BorderRadius.all(Radius.circular(7.0));
+    const BorderRadius enabledBorder = BorderRadius.all(Radius.circular(9.0));
+
+    final FocusNode focusNode = FocusNode();
+
+    const String errorText = 'This is an error';
+    bool showError = false;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(
+          inputDecorationTheme: const InputDecorationTheme(
+            errorBorder: OutlineInputBorder(
+              borderRadius: errorBorderRadius,
+            ),
+            focusedErrorBorder: OutlineInputBorder(
+              borderRadius: focusedErrorBorderRadius,
+            ),
+            focusedBorder: OutlineInputBorder(
+              borderRadius: focusedBorder,
+            ),
+            enabledBorder: OutlineInputBorder(
+              borderRadius: enabledBorder,
+            ),
+          ),
+        ),
+        home: Material(
+          child: Center(
+            child: StatefulBuilder(
+              builder: (BuildContext context, StateSetter setState) {
+                return DropdownButtonFormField<String>(
+                  value: 'two',
+                  onChanged:(String? value) {
+                    setState(() {
+                      if (value == 'three') {
+                        showError = true;
+                      } else {
+                        showError = false;
+                      }
+                    });
+                  },
+                  decoration: InputDecoration(
+                    errorText: showError ? errorText : null,
+                  ),
+                  focusNode: focusNode,
+                  items: menuItems.map<DropdownMenuItem<String>>((String item) {
+                    return DropdownMenuItem<String>(
+                      key: ValueKey<String>(item),
+                      value: item,
+                      child: Text(item, key: ValueKey<String>('${item}Text')),
+                    );
+                  }).toList(),
+                );
+              }
+            ),
+          ),
+        ),
+      ),
+    );
+
+    // Test enabled border.
+    InkWell inkWell = tester.widget<InkWell>(find.byType(InkWell));
+    expect(inkWell.borderRadius, enabledBorder);
+
+    // Test focused border.
+    focusNode.requestFocus();
+    await tester.pump();
+
+    inkWell = tester.widget<InkWell>(find.byType(InkWell));
+    expect(inkWell.borderRadius, focusedBorder);
+
+    // Test focused error border.
+    await tester.tap(find.text('two'), warnIfMissed: false);
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('three').last);
+    await tester.pumpAndSettle();
+
+    inkWell = tester.widget<InkWell>(find.byType(InkWell));
+    expect(inkWell.borderRadius, focusedErrorBorderRadius);
+
+    // Test error border with no focus.
+    focusNode.unfocus();
+    await tester.pump();
+
+    // Hovering over the widget should show the error border.
+    final TestGesture gesture = await tester.createGesture(kind: PointerDeviceKind.mouse);
+    await gesture.moveTo(tester.getCenter(find.text('three').last));
+    await tester.pumpAndSettle();
+
+    inkWell = tester.widget<InkWell>(find.byType(InkWell));
+    expect(inkWell.borderRadius, errorBorderRadius);
+  });
 }