CupertinoTextField to show disabled background color when decoration is null (#78140)

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

@@ -1216,6 +1216,7 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
         ignoring: !enabled,
         child: Container(
           decoration: effectiveDecoration,
+          color: !enabled && effectiveDecoration == null ? disabledColor : null,
           child: _selectionGestureDetectorBuilder.buildGestureDetector(
             behavior: HitTestBehavior.translucent,
             child: Align(

packages/flutter/test/cupertino/text_field_test.dart

@@ -4695,4 +4695,28 @@ void main() {
       0xFF050505,
     );
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/78097.
+  testWidgets(
+    'still gets disabled background color when decoration is null',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const CupertinoApp(
+          home: Center(
+            child: CupertinoTextField(
+              decoration: null,
+              enabled: false,
+            ),
+          ),
+        ),
+      );
+
+      final Color disabledColor = tester.widget<ColoredBox>(
+        find.descendant(
+          of: find.byType(CupertinoTextField),
+          matching: find.byType(ColoredBox),
+        ),
+      ).color;
+      expect(disabledColor, isSameColorAs(const Color(0xFFFAFAFA)));
+  });
 }