Make `CupertinoTextField` at least as tall as its first line of placeholder (#134198)

Fixes https://github.com/flutter/flutter/issues/133241
and some CupertinoTextField cleanup.

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

@@ -444,7 +444,7 @@ class _CupertinoSearchTextFieldState extends State<CupertinoSearchTextField>
       suffix: suffix,
       keyboardType: widget.keyboardType,
       onTap: widget.onTap,
-      enabled: widget.enabled,
+      enabled: widget.enabled ?? true,
       suffixMode: widget.suffixMode,
       placeholder: placeholder,
       placeholderStyle: placeholderStyle,

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

@@ -219,7 +219,7 @@ class CupertinoTextFormFieldRow extends FormField<String> {
                 onEditingComplete: onEditingComplete,
                 onSubmitted: onFieldSubmitted,
                 inputFormatters: inputFormatters,
-                enabled: enabled,
+                enabled: enabled ?? true,
                 cursorWidth: cursorWidth,
                 cursorHeight: cursorHeight,
                 cursorColor: cursorColor,

packages/flutter/lib/src/rendering/stack.dart

@@ -569,15 +569,11 @@ class RenderStack extends RenderBox
     double width = constraints.minWidth;
     double height = constraints.minHeight;
 
-    final BoxConstraints nonPositionedConstraints;
-    switch (fit) {
-      case StackFit.loose:
-        nonPositionedConstraints = constraints.loosen();
-      case StackFit.expand:
-        nonPositionedConstraints = BoxConstraints.tight(constraints.biggest);
-      case StackFit.passthrough:
-        nonPositionedConstraints = constraints;
-    }
+    final BoxConstraints nonPositionedConstraints = switch (fit) {
+      StackFit.loose => constraints.loosen(),
+      StackFit.expand => BoxConstraints.tight(constraints.biggest),
+      StackFit.passthrough => constraints,
+    };
 
     RenderBox? child = firstChild;
     while (child != null) {

packages/flutter/test/cupertino/text_field_test.dart

@@ -1071,7 +1071,8 @@ void main() {
 
       await tester.enterText(find.byType(CupertinoTextField), 'input');
       await tester.pump();
-      expect(find.text('placeholder'), findsNothing);
+      final Element element = tester.element(find.text('placeholder'));
+      expect(Visibility.of(element), false);
     },
   );
 
@@ -1964,7 +1965,9 @@ void main() {
 
     expect(find.text('field 1'), findsOneWidget);
     expect(find.text("j'aime la poutine"), findsOneWidget);
-    expect(find.text('field 2'), findsNothing);
+
+    final Element placeholder2Element = tester.element(find.text('field 2'));
+    expect(Visibility.of(placeholder2Element), false);
   }, skip: isContextMenuProvidedByPlatform); // [intended] only applies to platforms where we supply the context menu.
 
   testWidgets(
@@ -8096,9 +8099,7 @@ void main() {
     await tester.pumpWidget(
       const CupertinoApp(
         home: Center(
-          child: CupertinoTextField(
-            enabled: true,
-          ),
+          child: CupertinoTextField(),
         ),
       ),
     );
@@ -9867,4 +9868,59 @@ void main() {
     skip: isContextMenuProvidedByPlatform, // [intended] only applies to platforms where we supply the context menu.
     variant: TargetPlatformVariant.all(excluding: <TargetPlatform>{ TargetPlatform.iOS }),
   );
+
+  testWidgets('Does not shrink in height when enters text when there is large single-line placeholder', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/133241.
+    final TextEditingController controller = TextEditingController();
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: Align(
+          alignment: Alignment.topCenter,
+          child: CupertinoTextField(
+            placeholderStyle: const TextStyle(fontSize: 100),
+            placeholder: 'p',
+            controller: controller,
+          ),
+        ),
+      ),
+    );
+
+    final Rect rectWithPlaceholder = tester.getRect(find.byType(CupertinoTextField));
+    controller.value = const TextEditingValue(text: 'input');
+    await tester.pump();
+
+    final Rect rectWithText = tester.getRect(find.byType(CupertinoTextField));
+    expect(rectWithPlaceholder, rectWithText);
+  });
+
+  testWidgets('Does not match the height of a multiline placeholder', (WidgetTester tester) async {
+    final TextEditingController controller = TextEditingController();
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: Align(
+          alignment: Alignment.topCenter,
+          child: CupertinoTextField(
+            placeholderStyle: const TextStyle(fontSize: 100),
+            placeholder: 'p' * 50,
+            maxLines: null,
+            controller: controller,
+          ),
+        ),
+      ),
+    );
+
+    final Rect rectWithPlaceholder = tester.getRect(find.byType(CupertinoTextField));
+    controller.value = const TextEditingValue(text: 'input');
+    await tester.pump();
+
+    final Rect rectWithText = tester.getRect(find.byType(CupertinoTextField));
+    // The text field is still top aligned.
+    expect(rectWithPlaceholder.top, rectWithText.top);
+    // But after entering text the text field should shrink since the
+    // placeholder text is huge and multiline.
+    expect(rectWithPlaceholder.height, greaterThan(rectWithText.height));
+    // But still should be taller than or the same height of the first line of
+    // placeholder.
+    expect(rectWithText.height, greaterThan(100));
+  });
 }