CupertinoSearchTextField and CupertinoSliverNavigationBar.search more fidelity updates (#169708)

This PR does the following:

## Show large title mid-transition if automaticallyImplyLeading is false

(see
https://github.com/flutter/flutter/pull/169708#issuecomment-2922792587)

## Correct color for the CupertinoSearchTextField placeholder 

(see
https://github.com/flutter/flutter/issues/163020#issuecomment-2660522169)

| Dark mode | Light mode | 
| --- | --- |
| <img width="381" alt="placeholder color dark mode"
src="https://github.com/user-attachments/assets/e37e23fa-9f4f-495e-8f02-b9c38a4faffb"
/> | <img width="381" alt="placeholder color light mode"
src="https://github.com/user-attachments/assets/16e24a61-2528-44e0-9afa-8431487cf5ff"
/> |

And also:

- Removes flaky mid-transition goldens
- Fixes a CupertinoTextField crash caused by
https://github.com/flutter/flutter/pull/166952 where size > constraints

Fixes [Improve fidelity of CupertinoSliverNavigationBar.search and
CupertinoSearchTextField](https://github.com/flutter/flutter/issues/163020)

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

@@ -883,12 +883,19 @@ class _CupertinoNavigationBarState extends State<CupertinoNavigationBar> {
 /// It should be placed at top of the screen and automatically accounts for
 /// the iOS status bar.
 ///
+/// This navigation bar is expanded only in portrait orientation. In landscape
+/// mode, the navigation bar remains permanently collapsed. The navigation bar
+/// also collapses when scrolling in portrait mode.
+///
 /// Minimally, a [largeTitle] widget will appear in the middle of the app bar
 /// when the sliver is collapsed and transfer to the area below in larger font
-/// when the sliver is expanded.
+/// when the sliver is expanded. This expanded view will only trigger in
+/// portrait orientation, while in landscape mode the bar will stay in its
+/// collapsed view.
 ///
-/// For advanced uses, an optional [middle] widget can be supplied to show a
-/// different widget in the middle of the navigation bar when the sliver is collapsed.
+/// For advanced uses, an optional [middle] widget
+/// can be supplied to show a different widget in the middle of the navigation
+/// bar when the sliver is collapsed.
 ///
 /// Like [CupertinoNavigationBar], it also supports a [leading] and [trailing]
 /// widget on the static section on top that remains while scrolling.
@@ -1085,10 +1092,12 @@ class CupertinoSliverNavigationBar extends StatefulWidget {
   /// A widget to place in the middle of the static navigation bar instead of
   /// the [largeTitle].
   ///
-  /// This widget is visible in both collapsed and expanded states if
-  /// [alwaysShowMiddle] is true, otherwise just in collapsed state. The text
-  /// supplied in [largeTitle] will no longer appear in collapsed state if a
-  /// [middle] widget is provided.
+  /// If [alwaysShowMiddle] is true, this widget is visible in both the
+  /// collapsed and expanded states of the navigation bar. Else, it is visible
+  /// only in the collapsed state.
+  ///
+  /// If null, [largeTitle] will be displayed in the navigation bar's collapsed
+  /// state.
   final Widget? middle;
 
   /// {@macro flutter.cupertino.CupertinoNavigationBar.trailing}
@@ -3060,7 +3069,6 @@ class _NavigationBarComponentsTransition {
     final KeyedSubtree? bottomLargeTitle =
         bottomComponents.largeTitleKey.currentWidget as KeyedSubtree?;
     final KeyedSubtree? topBackLabel = topComponents.backLabelKey.currentWidget as KeyedSubtree?;
-    final KeyedSubtree? topLeading = topComponents.leadingKey.currentWidget as KeyedSubtree?;
 
     if (bottomLargeTitle == null || !bottomLargeExpanded) {
       return null;
@@ -3093,32 +3101,28 @@ class _NavigationBarComponentsTransition {
       );
     }
 
-    if (topLeading != null) {
-      // Unlike bottom middle, the bottom large title moves when it can't
-      // transition to the top back label position.
-      final RelativeRect from = positionInTransitionBox(
-        bottomComponents.largeTitleKey,
-        from: bottomNavBarBox,
-      );
+    // Unlike bottom middle, the bottom large title moves when it can't
+    // transition to the top back label position.
+    final RelativeRect from = positionInTransitionBox(
+      bottomComponents.largeTitleKey,
+      from: bottomNavBarBox,
+    );
 
-      final RelativeRectTween positionTween = RelativeRectTween(
-        begin: from,
-        end: from.shift(Offset(forwardDirection * bottomNavBarBox.size.width / 4.0, 0.0)),
-      );
+    final RelativeRectTween positionTween = RelativeRectTween(
+      begin: from,
+      end: from.shift(Offset(forwardDirection * bottomNavBarBox.size.width / 4.0, 0.0)),
+    );
 
-      // Just shift slightly towards the trailing edge instead of moving to the
-      // back label position.
-      return PositionedTransition(
-        rect: animation.drive(positionTween),
-        child: FadeTransition(
-          opacity: fadeOutBy(0.4),
-          // Keep the font when transitioning into a non-back-label leading.
-          child: DefaultTextStyle(style: bottomLargeTitleTextStyle!, child: bottomLargeTitle.child),
-        ),
-      );
-    }
-
-    return null;
+    // Just shift slightly towards the trailing edge instead of moving to the
+    // back label position.
+    return PositionedTransition(
+      rect: animation.drive(positionTween),
+      child: FadeTransition(
+        opacity: fadeOutBy(0.4),
+        // Keep the font when transitioning into a non-back-label leading.
+        child: DefaultTextStyle(style: bottomLargeTitleTextStyle!, child: bottomLargeTitle.child),
+      ),
+    );
   }
 
   Widget? get bottomTrailing {

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

@@ -483,10 +483,17 @@ class _CupertinoSearchTextFieldState extends State<CupertinoSearchTextField> wit
   Widget build(BuildContext context) {
     final String placeholder =
         widget.placeholder ?? CupertinoLocalizations.of(context).searchTextFieldPlaceholderLabel;
-
+    final Color defaultPlaceholderColor = CupertinoDynamicColor.resolve(
+      CupertinoColors.secondaryLabel,
+      context,
+    );
     final TextStyle placeholderStyle =
         widget.placeholderStyle ??
-        TextStyle(color: CupertinoColors.systemGrey.withOpacity(1.0 - _fadeExtent));
+        TextStyle(
+          color: defaultPlaceholderColor.withAlpha(
+            (255 * (defaultPlaceholderColor.a * (1 - _fadeExtent))).round(),
+          ),
+        );
 
     // The icon size will be scaled by a factor of the accessibility text scale,
     // to follow the behavior of `UISearchTextField`.

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

@@ -1879,6 +1879,6 @@ class _RenderBaselineAlignedStack extends RenderBox
     width = math.max(width, editableTextSize.width);
     final Size size = Size(width, height);
     assert(size.isFinite);
-    return size;
+    return constraints.constrain(size);
   }
 }

packages/flutter/test/cupertino/nav_bar_test.dart

@@ -2846,14 +2846,6 @@ void main() {
     // Tap the search field.
     await tester.tap(find.byType(CupertinoSearchTextField), warnIfMissed: false);
     await tester.pump();
-    // Pump halfway through the animation.
-    await tester.pump(const Duration(milliseconds: 150));
-
-    await expectLater(
-      find.byType(CupertinoSliverNavigationBar),
-      matchesGoldenFile('nav_bar.search.transition_forward.png'),
-    );
-
     // Pump to the end of the animation.
     await tester.pump(const Duration(milliseconds: 300));
 
@@ -2865,14 +2857,6 @@ void main() {
     // Tap the 'Cancel' button to exit the search view.
     await tester.tap(find.widgetWithText(CupertinoButton, 'Cancel'));
     await tester.pump();
-    // Pump halfway through the animation.
-    await tester.pump(const Duration(milliseconds: 150));
-
-    await expectLater(
-      find.byType(CupertinoSliverNavigationBar),
-      matchesGoldenFile('nav_bar.search.transition_backward.png'),
-    );
-
     // Pump for the duration of the search field animation.
     await tester.pump(const Duration(milliseconds: 300));
 

packages/flutter/test/cupertino/nav_bar_transition_test.dart

@@ -1850,6 +1850,33 @@ void main() {
     expect(find.text('Page 2'), findsOneWidget);
   });
 
+  testWidgets('Bottom large title is shown mid-transition when top has no leading', (
+    WidgetTester tester,
+  ) async {
+    setWindowToPortrait(tester);
+    await startTransitionBetween(
+      tester,
+      from: const CupertinoSliverNavigationBar(largeTitle: Text('Page 1')),
+      to: const CupertinoSliverNavigationBar(
+        largeTitle: Text('Page 2'),
+        automaticallyImplyLeading: false,
+      ),
+    );
+
+    // Go to the next page.
+    await tester.pump(const Duration(milliseconds: 600));
+
+    // Start the gesture at the edge of the screen.
+    final TestGesture gesture = await tester.startGesture(const Offset(5.0, 200.0));
+    // Trigger the swipe.
+    await gesture.moveBy(const Offset(200.0, 0.0));
+
+    // Back gestures should trigger and draw the hero transition in the very same
+    // frame (since the "from" route has already moved to reveal the "to" route).
+    await tester.pump();
+    expect(flying(tester, find.text('Page 1')), findsOneWidget);
+  });
+
   testWidgets('Back label is not clipped mid-transition', (WidgetTester tester) async {
     const String label = 'backbackback';
     await startTransitionBetween(

packages/flutter/test/cupertino/search_field_test.dart

@@ -129,7 +129,7 @@ void main() {
     );
 
     Text placeholder = tester.widget(find.text('Search'));
-    expect(placeholder.style!.color!.value, CupertinoColors.systemGrey.darkColor.value);
+    expect(placeholder.style!.color!.value, CupertinoColors.secondaryLabel.darkColor.value);
 
     await tester.pumpAndSettle();
 
@@ -141,7 +141,7 @@ void main() {
     );
 
     placeholder = tester.widget(find.text('Search'));
-    expect(placeholder.style!.color!.value, CupertinoColors.systemGrey.color.value);
+    expect(placeholder.style!.color!.value, CupertinoColors.secondaryLabel.color.value);
   });
 
   testWidgets("placeholderStyle modifies placeholder's style and doesn't affect text's style", (
@@ -623,7 +623,7 @@ void main() {
     expect(suffixIconFinder, findsOneWidget);
     expect(placeholderFinder, findsOneWidget);
 
-    // Initially, the icons and placeholder text are fully opaque.
+    // Initially, the icons are fully opaque.
     expect(
       tester
           .widget<Opacity>(find.ancestor(of: prefixIconFinder, matching: find.byType(Opacity)))
@@ -636,7 +636,8 @@ void main() {
           .opacity,
       equals(1.0),
     );
-    expect(tester.widget<Text>(placeholderFinder).style?.color?.a, equals(1.0));
+    // The default placeholder color is semi-transparent.
+    expect(tester.widget<Text>(placeholderFinder).style?.color?.a, equals(0.6));
 
     final double searchTextFieldHeight = tester.getSize(searchTextFieldFinder).height;