Overlay supports unconstrained environments (#139513)

Fixes https://github.com/flutter/flutter/issues/137875.

Unfortunately, we cannot auto-detect which OverlayEntry should be sizing the Overlay in unconstrained environment. So, this PR adds a special flag to annotate the Overlay Entry that should be used.

packages/flutter/lib/src/widgets/overlay.dart

@@ -82,6 +82,7 @@ class OverlayEntry implements Listenable {
     required this.builder,
     bool opaque = false,
     bool maintainState = false,
+    this.canSizeOverlay = false,
   }) : _opaque = opaque,
        _maintainState = maintainState {
     if (kFlutterMemoryAllocationsEnabled) {
@@ -138,6 +139,25 @@ class OverlayEntry implements Listenable {
     _overlay!._didChangeEntryOpacity();
   }
 
+  /// Whether the content of this [OverlayEntry] can be used to size the
+  /// [Overlay].
+  ///
+  /// In most situations the overlay sizes itself based on its incoming
+  /// constraints to be as large as possible. However, if that would result in
+  /// an infinite size, it has to rely on one of its children to size itself. In
+  /// this situation, the overlay will consult the topmost non-[Positioned]
+  /// overlay entry that has this property set to true, lay it out with the
+  /// incoming [BoxConstraints] of the overlay, and force all other
+  /// non-[Positioned] overlay entries to have the same size. The [Positioned]
+  /// entries are laid out as usual based on the calculated size of the overlay.
+  ///
+  /// Overlay entries that set this to true must be able to handle unconstrained
+  /// [BoxConstraints].
+  ///
+  /// Setting this to true has no effect if the overlay entry uses a [Positioned]
+  /// widget to position itself in the overlay.
+  final bool canSizeOverlay;
+
   /// Whether the [OverlayEntry] is currently mounted in the widget tree.
   ///
   /// The [OverlayEntry] notifies its listeners when this value changes.
@@ -830,6 +850,7 @@ class _WrappingOverlay extends StatefulWidget {
 
 class _WrappingOverlayState extends State<_WrappingOverlay> {
   late final OverlayEntry _entry = OverlayEntry(
+    canSizeOverlay: true,
     opaque: true,
     builder: (BuildContext context) {
       return widget.child;
@@ -940,28 +961,17 @@ mixin _RenderTheaterMixin on RenderBox {
     }
   }
 
-  @override
-  bool get sizedByParent => true;
-
-  @override
-  void performLayout() {
-    final Iterator<RenderBox> iterator = _childrenInPaintOrder().iterator;
-    // Same BoxConstraints as used by RenderStack for StackFit.expand.
-    final BoxConstraints nonPositionedChildConstraints = BoxConstraints.tight(constraints.biggest);
+  void layoutChild(RenderBox child, BoxConstraints nonPositionedChildConstraints) {
+    final StackParentData childParentData = child.parentData! as StackParentData;
     final Alignment alignment = theater._resolvedAlignment;
-
-    while (iterator.moveNext()) {
-      final RenderBox child = iterator.current;
-      final StackParentData childParentData = child.parentData! as StackParentData;
-      if (!childParentData.isPositioned) {
-        child.layout(nonPositionedChildConstraints, parentUsesSize: true);
-        childParentData.offset = alignment.alongOffset(size - child.size as Offset);
-      } else {
-        assert(child is! _RenderDeferredLayoutBox, 'all _RenderDeferredLayoutBoxes must be non-positioned children.');
-        RenderStack.layoutPositionedChild(child, childParentData, size, alignment);
-      }
-      assert(child.parentData == childParentData);
+    if (!childParentData.isPositioned) {
+      child.layout(nonPositionedChildConstraints, parentUsesSize: true);
+      childParentData.offset = Offset.zero;
+    } else {
+      assert(child is! _RenderDeferredLayoutBox, 'all _RenderDeferredLayoutBoxes must be non-positioned children.');
+      RenderStack.layoutPositionedChild(child, childParentData, size, alignment);
     }
+    assert(child.parentData == childParentData);
   }
 
   @override
@@ -1208,8 +1218,10 @@ class _RenderTheater extends RenderBox with ContainerRenderObjectMixin<RenderBox
 
   @override
   Size computeDryLayout(BoxConstraints constraints) {
-    assert(constraints.biggest.isFinite);
-    return constraints.biggest;
+    if (constraints.biggest.isFinite) {
+      return constraints.biggest;
+    }
+    return _findSizeDeterminingChild().getDryLayout(constraints);
   }
 
   @override
@@ -1249,6 +1261,53 @@ class _RenderTheater extends RenderBox with ContainerRenderObjectMixin<RenderBox
     }
   }
 
+  @override
+  bool get sizedByParent => false;
+
+  @override
+  void performLayout() {
+    RenderBox? sizeDeterminingChild;
+    if (constraints.biggest.isFinite) {
+      size = constraints.biggest;
+    } else {
+      sizeDeterminingChild = _findSizeDeterminingChild();
+      layoutChild(sizeDeterminingChild, constraints);
+      size = sizeDeterminingChild.size;
+    }
+
+    // Equivalent to BoxConstraints used by RenderStack for StackFit.expand.
+    final BoxConstraints nonPositionedChildConstraints = BoxConstraints.tight(size);
+    for (final RenderBox child in _childrenInPaintOrder()) {
+      if (child != sizeDeterminingChild) {
+        layoutChild(child, nonPositionedChildConstraints);
+      }
+    }
+  }
+
+  RenderBox _findSizeDeterminingChild() {
+    RenderBox? child = _lastOnstageChild;
+    while (child != null) {
+      final _TheaterParentData childParentData = child.parentData! as _TheaterParentData;
+      if ((childParentData.overlayEntry?.canSizeOverlay ?? false) && !childParentData.isPositioned) {
+        return child;
+      }
+      child = childParentData.previousSibling;
+    }
+    throw FlutterError.fromParts(<DiagnosticsNode>[
+      ErrorSummary('Overlay was given infinite constraints and cannot be sized by a suitable child.'),
+      ErrorDescription(
+        'The constraints given to the overlay ($constraints) would result in an illegal '
+        'infinite size (${constraints.biggest}). To avoid that, the Overlay tried to size '
+        'itself to one of its children, but no suitable non-positioned child that belongs to an '
+        'OverlayEntry with canSizeOverlay set to true could be found.',
+      ),
+      ErrorHint(
+        'Try wrapping the Overlay in a SizedBox to give it a finite size or '
+        'use an OverlayEntry with canSizeOverlay set to true.',
+      ),
+    ]);
+  }
+
   final LayerHandle<ClipRectLayer> _clipRectLayer = LayerHandle<ClipRectLayer>();
 
   @override
@@ -2185,6 +2244,9 @@ final class _RenderDeferredLayoutBox extends RenderProxyBox with _RenderTheaterM
     _callingMarkParentNeedsLayout = false;
   }
 
+  @override
+  bool get sizedByParent => true;
+
   bool _needsLayout = true;
   @override
   void markNeedsLayout() {
@@ -2254,7 +2316,8 @@ final class _RenderDeferredLayoutBox extends RenderProxyBox with _RenderTheaterM
       _needsLayout = false;
       return;
     }
-    super.performLayout();
+    assert(constraints.isTight);
+    layoutChild(child, constraints);
     assert(() {
       _debugMutationsLocked = false;
       return true;

packages/flutter/lib/src/widgets/routes.dart

@@ -1847,7 +1847,7 @@ abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T
   Iterable<OverlayEntry> createOverlayEntries() {
     return <OverlayEntry>[
       _modalBarrier = OverlayEntry(builder: _buildModalBarrier),
-      _modalScope = OverlayEntry(builder: _buildModalScope, maintainState: maintainState),
+      _modalScope = OverlayEntry(builder: _buildModalScope, maintainState: maintainState, canSizeOverlay: opaque),
     ];
   }
 

packages/flutter/test/material/app_test.dart

@@ -1602,6 +1602,19 @@ void main() {
     expect(find.byType(AnimatedTheme), findsNothing);
     expect(find.byType(Theme), findsOneWidget);
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/137875.
+  testWidgets('MaterialApp works in an unconstrained environment', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const UnconstrainedBox(
+        child: MaterialApp(
+          home: SizedBox(width: 123, height: 456),
+        ),
+      ),
+    );
+
+    expect(tester.getSize(find.byType(MaterialApp)), const Size(123, 456));
+  });
 }
 
 class MockScrollBehavior extends ScrollBehavior {

packages/flutter/test/widgets/overlay_test.dart

@@ -1593,6 +1593,177 @@ void main() {
     expect(find.text('Bye, bye'), findsOneWidget);
     expect(tester.state(find.byType(Overlay)), same(overlayState));
   });
+
+  testWidgets('Overlay.wrap is sized by child in an unconstrained environment', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: UnconstrainedBox(
+          child: Overlay.wrap(
+            child: const Center(
+              child: SizedBox(
+                width: 123,
+                height: 456,
+              )
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.getSize(find.byType(Overlay)), const Size(123, 456));
+  });
+
+  testWidgets('Overlay is sized by child in an unconstrained environment', (WidgetTester tester) async {
+    final OverlayEntry initialEntry = OverlayEntry(
+      opaque: true,
+      canSizeOverlay: true,
+      builder: (BuildContext context) {
+        return const SizedBox(width: 123, height: 456);
+      }
+    );
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: UnconstrainedBox(
+          child: Overlay(
+            initialEntries: <OverlayEntry>[initialEntry]
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.getSize(find.byType(Overlay)), const Size(123, 456));
+
+    final OverlayState overlay = tester.state<OverlayState>(find.byType(Overlay));
+
+    final OverlayEntry nonSizingEntry = OverlayEntry(
+      builder: (BuildContext context) {
+        return const SizedBox(
+          width: 600,
+          height: 600,
+          child: Center(child: Text('Hello')),
+        );
+      },
+    );
+
+    overlay.insert(nonSizingEntry);
+    await tester.pump();
+    expect(tester.getSize(find.byType(Overlay)), const Size(123, 456));
+    expect(find.text('Hello'), findsOneWidget);
+
+    final OverlayEntry sizingEntry = OverlayEntry(
+      canSizeOverlay: true,
+      builder: (BuildContext context) {
+        return const SizedBox(
+          width: 222,
+          height: 111,
+          child: Center(child: Text('World')),
+        );
+      },
+    );
+
+    overlay.insert(sizingEntry);
+    await tester.pump();
+    expect(tester.getSize(find.byType(Overlay)), const Size(222, 111));
+    expect(find.text('Hello'), findsOneWidget);
+    expect(find.text('World'), findsOneWidget);
+
+    nonSizingEntry.remove();
+    await tester.pump();
+    expect(tester.getSize(find.byType(Overlay)), const Size(222, 111));
+    expect(find.text('Hello'), findsNothing);
+    expect(find.text('World'), findsOneWidget);
+
+    sizingEntry.remove();
+    await tester.pump();
+    expect(tester.getSize(find.byType(Overlay)), const Size(123, 456));
+    expect(find.text('Hello'), findsNothing);
+    expect(find.text('World'), findsNothing);
+  });
+
+  testWidgets('Overlay throws if unconstrained and has no child', (WidgetTester tester) async {
+    final List<FlutterErrorDetails> errors = <FlutterErrorDetails>[];
+    final FlutterExceptionHandler? oldHandler = FlutterError.onError;
+    FlutterError.onError = errors.add;
+
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: UnconstrainedBox(
+          child: Overlay(),
+        ),
+      ),
+    );
+    FlutterError.onError = oldHandler;
+
+    expect(
+      errors.first.toString().replaceAll('\n', ' '),
+      contains('Overlay was given infinite constraints and cannot be sized by a suitable child.'),
+    );
+  });
+
+  testWidgets('Overlay throws if unconstrained and only positioned child', (WidgetTester tester) async {
+    final List<FlutterErrorDetails> errors = <FlutterErrorDetails>[];
+    final FlutterExceptionHandler? oldHandler = FlutterError.onError;
+    FlutterError.onError = errors.add;
+
+    final OverlayEntry entry = OverlayEntry(
+      canSizeOverlay: true,
+      builder: (BuildContext context) {
+        return const Positioned(
+          top: 100,
+          child: SizedBox(width: 600, height: 600),
+        );
+      },
+    );
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: UnconstrainedBox(
+          child: Overlay(
+            initialEntries: <OverlayEntry>[entry],
+          ),
+        ),
+      ),
+    );
+    FlutterError.onError = oldHandler;
+
+    expect(
+      errors.first.toString().replaceAll('\n', ' '),
+      contains('Overlay was given infinite constraints and cannot be sized by a suitable child.'),
+    );
+  });
+
+  testWidgets('Overlay throws if unconstrained and no canSizeOverlay child', (WidgetTester tester) async {
+    final List<FlutterErrorDetails> errors = <FlutterErrorDetails>[];
+    final FlutterExceptionHandler? oldHandler = FlutterError.onError;
+    FlutterError.onError = errors.add;
+
+    final OverlayEntry entry = OverlayEntry(
+      builder: (BuildContext context) {
+        return const SizedBox(width: 600, height: 600);
+      },
+    );
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: UnconstrainedBox(
+          child: Overlay(
+            initialEntries: <OverlayEntry>[entry],
+          ),
+        ),
+      ),
+    );
+    FlutterError.onError = oldHandler;
+
+    expect(
+      errors.first.toString().replaceAll('\n', ' '),
+      contains('Overlay was given infinite constraints and cannot be sized by a suitable child.'),
+    );
+  });
 }
 
 class StatefulTestWidget extends StatefulWidget {