Make CupertinoTextField respect decoration color when disabled (#149774)

#83531 contains complaints that this default disabled color can no longer be overridden by setting a color of the `decoration` property. This regression was introduced by #78058, which was itself made to fix an issue where the color of the decoration was always used even when disabled.

Currently, when the text field is disabled, its color is set to a default disabled color. This PR makes it so that if a decoration is set when the text field is disabled, its color is used instead of the default disabled color.

Fixes flutter/flutter#83531

Author: victorsanni

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

@@ -1338,10 +1338,16 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
         );
     }
 
-    final BoxDecoration? effectiveDecoration = widget.decoration?.copyWith(
-      border: resolvedBorder,
-      color: enabled ? decorationColor : disabledColor,
-    );
+    // Use the default disabled color only if the box decoration was not set.
+    final BoxDecoration? effectiveDecoration =
+      widget.decoration?.copyWith(
+        border: resolvedBorder,
+        color: enabled ? decorationColor
+          : (widget.decoration == _kDefaultRoundedBorderDecoration
+              ? disabledColor
+              : widget.decoration?.color
+            ),
+      );
 
     final Color selectionColor = CupertinoDynamicColor.maybeResolve(
       DefaultSelectionStyle.of(context).selectionColor,

packages/flutter/test/cupertino/text_field_test.dart

@@ -8397,6 +8397,34 @@ void main() {
     );
   });
 
+  testWidgets('Disabled widget does not override background color', (WidgetTester tester) async {
+    const Color backgroundColor = Color(0x0000000A);
+    await tester.pumpWidget(
+      const CupertinoApp(
+        home: Center(
+          child: CupertinoTextField(
+            enabled: false,
+            decoration: BoxDecoration(color: backgroundColor),
+          ),
+        ),
+      ),
+    );
+
+    final BoxDecoration decoration = tester
+        .widget<DecoratedBox>(
+          find.descendant(
+            of: find.byType(CupertinoTextField),
+            matching: find.byType(DecoratedBox),
+          ),
+        )
+        .decoration as BoxDecoration;
+
+    expect(
+      decoration.color!.value,
+      backgroundColor.value,
+    );
+  });
+
   // Regression test for https://github.com/flutter/flutter/issues/78097.
   testWidgets(
     'still gets disabled background color when decoration is null',