Fix disabled CupertinoTextField style (#32974)

bin/internal/goldens.version

-d79941e3d9c6e761189ac8aefa3a40c7fb98413e
+2eea0c710c4fca4c6655b4efc15b68d4b44cd404

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

@@ -785,6 +785,13 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with AutomaticK
     final TextStyle placeholderStyle = textStyle.merge(widget.placeholderStyle);
     final Brightness keyboardAppearance = widget.keyboardAppearance ?? themeData.brightness;
     final Color cursorColor = widget.cursorColor ?? themeData.primaryColor;
+    final Color disabledColor = CupertinoTheme.of(context).brightness == Brightness.light
+      ? _kDisabledBackground
+      : CupertinoColors.darkBackgroundGray;
+
+    final BoxDecoration effectiveDecoration = enabled
+      ? widget.decoration
+      : widget.decoration?.copyWith(color: widget.decoration?.color ?? disabledColor);
 
     final Widget paddedEditable = TextSelectionGestureDetector(
       onTapDown: _handleTapDown,
@@ -855,16 +862,8 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with AutomaticK
       child: IgnorePointer(
         ignoring: !enabled,
         child: Container(
-          decoration: widget.decoration,
-          // The main decoration and the disabled scrim exists separately.
-          child: Container(
-            color: enabled
-                ? null
-                : CupertinoTheme.of(context).brightness == Brightness.light
-                    ? _kDisabledBackground
-                    : CupertinoColors.darkBackgroundGray,
-            child: _addTextDependentAttachments(paddedEditable, textStyle, placeholderStyle),
-          ),
+          decoration: effectiveDecoration,
+          child: _addTextDependentAttachments(paddedEditable, textStyle, placeholderStyle),
         ),
       ),
     );

packages/flutter/lib/src/painting/box_decoration.dart

@@ -97,6 +97,30 @@ class BoxDecoration extends Decoration {
          'gradient, but no color or gradient was provided.'
        );
 
+  /// Creates a copy of this object but with the given fields replaced with the
+  /// new values.
+  BoxDecoration copyWith({
+    Color color,
+    DecorationImage image,
+    BoxBorder border,
+    BorderRadiusGeometry borderRadius,
+    List<BoxShadow> boxShadow,
+    Gradient gradient,
+    BlendMode backgroundBlendMode,
+    BoxShape shape,
+  }) {
+    return BoxDecoration(
+      color: color ?? this.color,
+      image: image ?? this.image,
+      border: border ?? this.border,
+      borderRadius: borderRadius ?? this.borderRadius,
+      boxShadow: boxShadow ?? this.boxShadow,
+      gradient: gradient ?? this.gradient,
+      backgroundBlendMode: backgroundBlendMode ?? this.backgroundBlendMode,
+      shape: shape ?? this.shape,
+    );
+  }
+
   @override
   bool debugAssertIsValid() {
     assert(shape != BoxShape.circle ||

packages/flutter/test/cupertino/text_field_test.dart

@@ -2590,4 +2590,30 @@ void main() {
 
     debugDefaultTargetPlatformOverride = null;
   });
+
+  testWidgets('disabled state golden', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: DecoratedBox(
+          decoration: const BoxDecoration(color: Color(0xFFFFFFFF)),
+          child: Center(
+            child: SizedBox(
+              width: 200,
+              height: 200,
+              child: CupertinoTextField(
+                controller: TextEditingController(text: 'lorem'),
+                enabled: false,
+              ),
+            )
+          )
+        )
+      )
+    );
+
+    await expectLater(
+      find.byType(CupertinoTextField),
+      matchesGoldenFile('text_field_test.disabled.0.png'),
+      skip: !Platform.isLinux,
+    );
+  });
 }