Reland "Do not rebuild Routes when a new opaque Route is pushed on top" (#49376)

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

@@ -425,7 +425,8 @@ class RenderStack extends RenderBox
     }
   }
 
-  double _getIntrinsicDimension(double mainChildSizeGetter(RenderBox child)) {
+  /// Helper function for calculating the intrinsics metrics of a Stack.
+  static double getIntrinsicDimension(RenderBox firstChild, double mainChildSizeGetter(RenderBox child)) {
     double extent = 0.0;
     RenderBox child = firstChild;
     while (child != null) {
@@ -440,22 +441,22 @@ class RenderStack extends RenderBox
 
   @override
   double computeMinIntrinsicWidth(double height) {
-    return _getIntrinsicDimension((RenderBox child) => child.getMinIntrinsicWidth(height));
+    return getIntrinsicDimension(firstChild, (RenderBox child) => child.getMinIntrinsicWidth(height));
   }
 
   @override
   double computeMaxIntrinsicWidth(double height) {
-    return _getIntrinsicDimension((RenderBox child) => child.getMaxIntrinsicWidth(height));
+    return getIntrinsicDimension(firstChild, (RenderBox child) => child.getMaxIntrinsicWidth(height));
   }
 
   @override
   double computeMinIntrinsicHeight(double width) {
-    return _getIntrinsicDimension((RenderBox child) => child.getMinIntrinsicHeight(width));
+    return getIntrinsicDimension(firstChild, (RenderBox child) => child.getMinIntrinsicHeight(width));
   }
 
   @override
   double computeMaxIntrinsicHeight(double width) {
-    return _getIntrinsicDimension((RenderBox child) => child.getMaxIntrinsicHeight(width));
+    return getIntrinsicDimension(firstChild, (RenderBox child) => child.getMaxIntrinsicHeight(width));
   }
 
   @override
@@ -463,6 +464,57 @@ class RenderStack extends RenderBox
     return defaultComputeDistanceToHighestActualBaseline(baseline);
   }
 
+  /// Lays out the positioned `child` according to `alignment` within a Stack of `size`.
+  ///
+  /// Returns true when the child has visual overflow.
+  static bool layoutPositionedChild(RenderBox child, StackParentData childParentData, Size size, Alignment alignment) {
+    assert(childParentData.isPositioned);
+    assert(child.parentData == childParentData);
+
+    bool hasVisualOverflow = false;
+    BoxConstraints childConstraints = const BoxConstraints();
+
+    if (childParentData.left != null && childParentData.right != null)
+      childConstraints = childConstraints.tighten(width: size.width - childParentData.right - childParentData.left);
+    else if (childParentData.width != null)
+      childConstraints = childConstraints.tighten(width: childParentData.width);
+
+    if (childParentData.top != null && childParentData.bottom != null)
+      childConstraints = childConstraints.tighten(height: size.height - childParentData.bottom - childParentData.top);
+    else if (childParentData.height != null)
+      childConstraints = childConstraints.tighten(height: childParentData.height);
+
+    child.layout(childConstraints, parentUsesSize: true);
+
+    double x;
+    if (childParentData.left != null) {
+      x = childParentData.left;
+    } else if (childParentData.right != null) {
+      x = size.width - childParentData.right - child.size.width;
+    } else {
+      x = alignment.alongOffset(size - child.size as Offset).dx;
+    }
+
+    if (x < 0.0 || x + child.size.width > size.width)
+      hasVisualOverflow = true;
+
+    double y;
+    if (childParentData.top != null) {
+      y = childParentData.top;
+    } else if (childParentData.bottom != null) {
+      y = size.height - childParentData.bottom - child.size.height;
+    } else {
+      y = alignment.alongOffset(size - child.size as Offset).dy;
+    }
+
+    if (y < 0.0 || y + child.size.height > size.height)
+      hasVisualOverflow = true;
+
+    childParentData.offset = Offset(x, y);
+
+    return hasVisualOverflow;
+  }
+
   @override
   void performLayout() {
     _resolve();
@@ -527,45 +579,7 @@ class RenderStack extends RenderBox
       if (!childParentData.isPositioned) {
         childParentData.offset = _resolvedAlignment.alongOffset(size - child.size as Offset);
       } else {
-        BoxConstraints childConstraints = const BoxConstraints();
-
-        if (childParentData.left != null && childParentData.right != null)
-          childConstraints = childConstraints.tighten(width: size.width - childParentData.right - childParentData.left);
-        else if (childParentData.width != null)
-          childConstraints = childConstraints.tighten(width: childParentData.width);
-
-        if (childParentData.top != null && childParentData.bottom != null)
-          childConstraints = childConstraints.tighten(height: size.height - childParentData.bottom - childParentData.top);
-        else if (childParentData.height != null)
-          childConstraints = childConstraints.tighten(height: childParentData.height);
-
-        child.layout(childConstraints, parentUsesSize: true);
-
-        double x;
-        if (childParentData.left != null) {
-          x = childParentData.left;
-        } else if (childParentData.right != null) {
-          x = size.width - childParentData.right - child.size.width;
-        } else {
-          x = _resolvedAlignment.alongOffset(size - child.size as Offset).dx;
-        }
-
-        if (x < 0.0 || x + child.size.width > size.width)
-          _hasVisualOverflow = true;
-
-        double y;
-        if (childParentData.top != null) {
-          y = childParentData.top;
-        } else if (childParentData.bottom != null) {
-          y = size.height - childParentData.bottom - child.size.height;
-        } else {
-          y = _resolvedAlignment.alongOffset(size - child.size as Offset).dy;
-        }
-
-        if (y < 0.0 || y + child.size.height > size.height)
-          _hasVisualOverflow = true;
-
-        childParentData.offset = Offset(x, y);
+        _hasVisualOverflow = layoutPositionedChild(child, childParentData, size, _resolvedAlignment) || _hasVisualOverflow;
       }
 
       assert(child.parentData == childParentData);

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

@@ -4,13 +4,13 @@
 
 import 'dart:async';
 import 'dart:collection';
+import 'dart:math' as math;
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/scheduler.dart';
 
 import 'basic.dart';
-import 'debug.dart';
 import 'framework.dart';
 import 'ticker_provider.dart';
 
@@ -115,7 +115,7 @@ class OverlayEntry {
   }
 
   OverlayState _overlay;
-  final GlobalKey<_OverlayEntryState> _key = GlobalKey<_OverlayEntryState>();
+  final GlobalKey<_OverlayEntryWidgetState> _key = GlobalKey<_OverlayEntryWidgetState>();
 
   /// Remove this entry from the overlay.
   ///
@@ -152,21 +152,30 @@ class OverlayEntry {
   String toString() => '${describeIdentity(this)}(opaque: $opaque; maintainState: $maintainState)';
 }
 
-class _OverlayEntry extends StatefulWidget {
-  _OverlayEntry(this.entry)
-    : assert(entry != null),
-      super(key: entry._key);
+class _OverlayEntryWidget extends StatefulWidget {
+  const _OverlayEntryWidget({
+    @required Key key,
+    @required this.entry,
+    this.tickerEnabled = true,
+  }) : assert(key != null),
+       assert(entry != null),
+       assert(tickerEnabled != null),
+       super(key: key);
 
   final OverlayEntry entry;
+  final bool tickerEnabled;
 
   @override
-  _OverlayEntryState createState() => _OverlayEntryState();
+  _OverlayEntryWidgetState createState() => _OverlayEntryWidgetState();
 }
 
-class _OverlayEntryState extends State<_OverlayEntry> {
+class _OverlayEntryWidgetState extends State<_OverlayEntryWidget> {
   @override
   Widget build(BuildContext context) {
-    return widget.entry.builder(context);
+    return TickerMode(
+      enabled: widget.tickerEnabled,
+      child: widget.entry.builder(context),
+    );
   }
 
   void _markNeedsBuild() {
@@ -452,28 +461,32 @@ class OverlayState extends State<Overlay> with TickerProviderStateMixin {
 
   @override
   Widget build(BuildContext context) {
-    // These lists are filled backwards. For the offstage children that
-    // does not matter since they aren't rendered, but for the onstage
-    // children we reverse the list below before adding it to the tree.
-    final List<Widget> onstageChildren = <Widget>[];
-    final List<Widget> offstageChildren = <Widget>[];
+    // This list is filled backwards and then reversed below before
+    // it is added to the tree.
+    final List<Widget> children = <Widget>[];
     bool onstage = true;
+    int onstageCount = 0;
     for (int i = _entries.length - 1; i >= 0; i -= 1) {
       final OverlayEntry entry = _entries[i];
       if (onstage) {
-        onstageChildren.add(_OverlayEntry(entry));
+        onstageCount += 1;
+        children.add(_OverlayEntryWidget(
+          key: entry._key,
+          entry: entry,
+        ));
         if (entry.opaque)
           onstage = false;
       } else if (entry.maintainState) {
-        offstageChildren.add(TickerMode(enabled: false, child: _OverlayEntry(entry)));
+        children.add(_OverlayEntryWidget(
+          key: entry._key,
+          entry: entry,
+          tickerEnabled: false,
+        ));
       }
     }
     return _Theatre(
-      onstage: Stack(
-        fit: StackFit.expand,
-        children: onstageChildren.reversed.toList(growable: false),
-      ),
-      offstage: offstageChildren,
+      skipCount: children.length - onstageCount,
+      children: children.reversed.toList(growable: false),
     );
   }
 
@@ -486,36 +499,50 @@ class OverlayState extends State<Overlay> with TickerProviderStateMixin {
   }
 }
 
-/// A widget that has one [onstage] child which is visible, and one or more
-/// [offstage] widgets which are kept alive, and are built, but are not laid out
-/// or painted.
+/// Special version of a [Stack], that doesn't layout and render the first
+/// [skipCount] children.
 ///
-/// The onstage widget must be a [Stack].
-///
-/// For convenience, it is legal to use [Positioned] widgets around the offstage
-/// widgets.
-class _Theatre extends RenderObjectWidget {
+/// The first [skipCount] children are considered "offstage".
+class _Theatre extends MultiChildRenderObjectWidget {
   _Theatre({
-    this.onstage,
-    @required this.offstage,
-  }) : assert(offstage != null),
-       assert(!offstage.any((Widget child) => child == null));
-
-  final Stack onstage;
+    Key key,
+    this.skipCount = 0,
+    List<Widget> children = const <Widget>[],
+  }) : assert(skipCount != null),
+       assert(skipCount >= 0),
+       assert(children != null),
+       assert(children.length >= skipCount),
+       super(key: key, children: children);
 
-  final List<Widget> offstage;
+  final int skipCount;
 
   @override
   _TheatreElement createElement() => _TheatreElement(this);
 
   @override
-  _RenderTheatre createRenderObject(BuildContext context) => _RenderTheatre();
+  _RenderTheatre createRenderObject(BuildContext context) {
+    return _RenderTheatre(
+      skipCount: skipCount,
+      textDirection: Directionality.of(context),
+    );
+  }
+
+  @override
+  void updateRenderObject(BuildContext context, _RenderTheatre renderObject) {
+    renderObject
+      ..skipCount = skipCount
+      ..textDirection = Directionality.of(context);
+  }
+
+  @override
+  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+    super.debugFillProperties(properties);
+    properties.add(IntProperty('skipCount', skipCount));
+  }
 }
 
-class _TheatreElement extends RenderObjectElement {
-  _TheatreElement(_Theatre widget)
-    : assert(!debugChildrenHaveDuplicateKeys(widget, widget.offstage)),
-      super(widget);
+class _TheatreElement extends MultiChildRenderObjectElement {
+  _TheatreElement(_Theatre widget) : super(widget);
 
   @override
   _Theatre get widget => super.widget as _Theatre;
@@ -523,186 +550,268 @@ class _TheatreElement extends RenderObjectElement {
   @override
   _RenderTheatre get renderObject => super.renderObject as _RenderTheatre;
 
-  Element _onstage;
-  static final Object _onstageSlot = Object();
+  @override
+  void debugVisitOnstageChildren(ElementVisitor visitor) {
+    assert(children.length >= widget.skipCount);
+    children.skip(widget.skipCount).forEach(visitor);
+  }
+}
+
+class _RenderTheatre extends RenderBox with ContainerRenderObjectMixin<RenderBox, StackParentData> {
+  _RenderTheatre({
+    List<RenderBox> children,
+    @required TextDirection textDirection,
+    int skipCount = 0,
+  }) : assert(skipCount != null),
+       assert(skipCount >= 0),
+       assert(textDirection != null),
+       _textDirection = textDirection,
+       _skipCount = skipCount {
+    addAll(children);
+  }
 
-  List<Element> _offstage;
-  final Set<Element> _forgottenOffstageChildren = HashSet<Element>();
+  bool _hasVisualOverflow = false;
 
   @override
-  void insertChildRenderObject(RenderBox child, dynamic slot) {
-    assert(renderObject.debugValidateChild(child));
-    if (slot == _onstageSlot) {
-      assert(child is RenderStack);
-      renderObject.child = child as RenderStack;
-    } else {
-      assert(slot == null || slot is Element);
-      renderObject.insert(child, after: slot?.renderObject as RenderBox);
+  void setupParentData(RenderBox child) {
+    if (child.parentData is! StackParentData)
+      child.parentData = StackParentData();
+  }
+
+  Alignment _resolvedAlignment;
+
+  void _resolve() {
+    if (_resolvedAlignment != null)
+      return;
+    _resolvedAlignment = AlignmentDirectional.topStart.resolve(textDirection);
+  }
+
+  void _markNeedResolution() {
+    _resolvedAlignment = null;
+    markNeedsLayout();
+  }
+
+  TextDirection get textDirection => _textDirection;
+  TextDirection _textDirection;
+  set textDirection(TextDirection value) {
+    if (_textDirection == value)
+      return;
+    _textDirection = value;
+    _markNeedResolution();
+  }
+
+  int get skipCount => _skipCount;
+  int _skipCount;
+  set skipCount(int value) {
+    assert(value != null);
+    if (_skipCount != value) {
+      _skipCount = value;
+      markNeedsLayout();
     }
   }
 
-  @override
-  void moveChildRenderObject(RenderBox child, dynamic slot) {
-    if (slot == _onstageSlot) {
-      renderObject.remove(child);
-      assert(child is RenderStack);
-      renderObject.child = child as RenderStack;
-    } else {
-      assert(slot == null || slot is Element);
-      if (renderObject.child == child) {
-        renderObject.child = null;
-        renderObject.insert(child, after: slot?.renderObject as RenderBox);
-      } else {
-        renderObject.move(child, after: slot?.renderObject as RenderBox);
-      }
+  RenderBox get _firstOnstageChild {
+    if (skipCount == super.childCount) {
+      return null;
+    }
+    RenderBox child = super.firstChild;
+    for (int toSkip = skipCount; toSkip > 0; toSkip--) {
+      final StackParentData childParentData = child.parentData as StackParentData;
+      child = childParentData.nextSibling;
+      assert(child != null);
     }
+    return child;
   }
 
+  RenderBox get _lastOnstageChild => skipCount == super.childCount ? null : lastChild;
+
+  int get _onstageChildCount => childCount - skipCount;
+
   @override
-  void removeChildRenderObject(RenderBox child) {
-    if (renderObject.child == child) {
-      renderObject.child = null;
-    } else {
-      renderObject.remove(child);
-    }
+  double computeMinIntrinsicWidth(double height) {
+    return RenderStack.getIntrinsicDimension(_firstOnstageChild, (RenderBox child) => child.getMinIntrinsicWidth(height));
   }
 
   @override
-  void visitChildren(ElementVisitor visitor) {
-    if (_onstage != null)
-      visitor(_onstage);
-    for (final Element child in _offstage) {
-      if (!_forgottenOffstageChildren.contains(child))
-        visitor(child);
-    }
+  double computeMaxIntrinsicWidth(double height) {
+    return RenderStack.getIntrinsicDimension(_firstOnstageChild, (RenderBox child) => child.getMaxIntrinsicWidth(height));
   }
 
   @override
-  void debugVisitOnstageChildren(ElementVisitor visitor) {
-    if (_onstage != null)
-      visitor(_onstage);
+  double computeMinIntrinsicHeight(double width) {
+    return RenderStack.getIntrinsicDimension(_firstOnstageChild, (RenderBox child) => child.getMinIntrinsicHeight(width));
   }
 
   @override
-  bool forgetChild(Element child) {
-    if (child == _onstage) {
-      _onstage = null;
-    } else {
-      assert(_offstage.contains(child));
-      assert(!_forgottenOffstageChildren.contains(child));
-      _forgottenOffstageChildren.add(child);
-    }
-    return true;
+  double computeMaxIntrinsicHeight(double width) {
+    return RenderStack.getIntrinsicDimension(_firstOnstageChild, (RenderBox child) => child.getMaxIntrinsicHeight(width));
   }
 
   @override
-  void mount(Element parent, dynamic newSlot) {
-    super.mount(parent, newSlot);
-    _onstage = updateChild(_onstage, widget.onstage, _onstageSlot);
-    _offstage = List<Element>(widget.offstage.length);
-    Element previousChild;
-    for (int i = 0; i < _offstage.length; i += 1) {
-      final Element newChild = inflateWidget(widget.offstage[i], previousChild);
-      _offstage[i] = newChild;
-      previousChild = newChild;
+  double computeDistanceToActualBaseline(TextBaseline baseline) {
+    assert(!debugNeedsLayout);
+    double result;
+    RenderBox child = _firstOnstageChild;
+    while (child != null) {
+      assert(!child.debugNeedsLayout);
+      final StackParentData childParentData = child.parentData as StackParentData;
+      double candidate = child.getDistanceToActualBaseline(baseline);
+      if (candidate != null) {
+        candidate += childParentData.offset.dy;
+        if (result != null) {
+          result = math.min(result, candidate);
+        } else {
+          result = candidate;
+        }
+      }
+      child = childParentData.nextSibling;
     }
+    return result;
   }
 
   @override
-  void update(_Theatre newWidget) {
-    super.update(newWidget);
-    assert(widget == newWidget);
-    _onstage = updateChild(_onstage, widget.onstage, _onstageSlot);
-    _offstage = updateChildren(_offstage, widget.offstage, forgottenChildren: _forgottenOffstageChildren);
-    _forgottenOffstageChildren.clear();
+  bool get sizedByParent => true;
+
+  @override
+  void performResize() {
+    size = constraints.biggest;
+    assert(size.isFinite);
   }
-}
 
-// A render object which lays out and paints one subtree while keeping a list
-// of other subtrees alive but not laid out or painted (the "zombie" children).
-//
-// The subtree that is laid out and painted must be a [RenderStack].
-//
-// This class uses [StackParentData] objects for its parent data so that the
-// children of its primary subtree's stack can be moved to this object's list
-// of zombie children without changing their parent data objects.
-class _RenderTheatre extends RenderBox
-  with RenderObjectWithChildMixin<RenderStack>, RenderProxyBoxMixin<RenderStack>,
-       ContainerRenderObjectMixin<RenderBox, StackParentData> {
+  @override
+  void performLayout() {
+    _hasVisualOverflow = false;
+
+    if (_onstageChildCount == 0) {
+      return;
+    }
+
+    _resolve();
+    assert(_resolvedAlignment != null);
+
+    // Same BoxConstraints as used by RenderStack for StackFit.expand.
+    final BoxConstraints nonPositionedConstraints = BoxConstraints.tight(constraints.biggest);
+
+    RenderBox child = _firstOnstageChild;
+    while (child != null) {
+      final StackParentData childParentData = child.parentData as StackParentData;
+
+      if (!childParentData.isPositioned) {
+        child.layout(nonPositionedConstraints, parentUsesSize: true);
+        childParentData.offset = _resolvedAlignment.alongOffset(size - child.size as Offset);
+      } else {
+        _hasVisualOverflow = RenderStack.layoutPositionedChild(child, childParentData, size, _resolvedAlignment) || _hasVisualOverflow;
+      }
+
+      assert(child.parentData == childParentData);
+      child = childParentData.nextSibling;
+    }
+  }
 
   @override
-  void setupParentData(RenderObject child) {
-    if (child.parentData is! StackParentData)
-      child.parentData = StackParentData();
+  bool hitTestChildren(BoxHitTestResult result, { Offset position }) {
+    RenderBox child = _lastOnstageChild;
+    for (int i = 0; i < _onstageChildCount; i++) {
+      assert(child != null);
+      final StackParentData childParentData = child.parentData as StackParentData;
+      final bool isHit = result.addWithPaintOffset(
+        offset: childParentData.offset,
+        position: position,
+        hitTest: (BoxHitTestResult result, Offset transformed) {
+          assert(transformed == position - childParentData.offset);
+          return child.hitTest(result, position: transformed);
+        },
+      );
+      if (isHit)
+        return true;
+      child = childParentData.previousSibling;
+    }
+    return false;
   }
 
-  // Because both RenderObjectWithChildMixin and ContainerRenderObjectMixin
-  // define redepthChildren, visitChildren and debugDescribeChildren and don't
-  // call super, we have to define them again here to make sure the work of both
-  // is done.
-  //
-  // We chose to put ContainerRenderObjectMixin last in the inheritance chain so
-  // that we can call super to hit its more complex definitions of
-  // redepthChildren and visitChildren, and then duplicate the more trivial
-  // definition from RenderObjectWithChildMixin inline in our version here.
-  //
-  // This code duplication is suboptimal.
-  // TODO(ianh): Replace this with a better solution once https://github.com/dart-lang/sdk/issues/27100 is fixed
-  //
-  // For debugDescribeChildren we just roll our own because otherwise the line
-  // drawings won't really work as well.
+  @protected
+  void paintStack(PaintingContext context, Offset offset) {
+    RenderBox child = _firstOnstageChild;
+    while (child != null) {
+      final StackParentData childParentData = child.parentData as StackParentData;
+      context.paintChild(child, childParentData.offset + offset);
+      child = childParentData.nextSibling;
+    }
+  }
 
   @override
-  void redepthChildren() {
-    if (child != null)
-      redepthChild(child);
-    super.redepthChildren();
+  void paint(PaintingContext context, Offset offset) {
+    if (_hasVisualOverflow) {
+      context.pushClipRect(needsCompositing, offset, Offset.zero & size, paintStack);
+    } else {
+      paintStack(context, offset);
+    }
   }
 
   @override
-  void visitChildren(RenderObjectVisitor visitor) {
-    if (child != null)
+  void visitChildrenForSemantics(RenderObjectVisitor visitor) {
+    RenderBox child = _firstOnstageChild;
+    while (child != null) {
       visitor(child);
-    super.visitChildren(visitor);
+      final StackParentData childParentData = child.parentData as StackParentData;
+      child = childParentData.nextSibling;
+    }
   }
 
   @override
-  List<DiagnosticsNode> debugDescribeChildren() {
-    final List<DiagnosticsNode> children = <DiagnosticsNode>[
-      if (child != null) child.toDiagnosticsNode(name: 'onstage'),
-    ];
+  Rect describeApproximatePaintClip(RenderObject child) => _hasVisualOverflow ? Offset.zero & size : null;
 
-    if (firstChild != null) {
-      RenderBox child = firstChild;
+  @override
+  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+    super.debugFillProperties(properties);
+    properties.add(IntProperty('skipCount', skipCount));
+    properties.add(EnumProperty<TextDirection>('textDirection', textDirection));
+  }
 
-      int count = 1;
-      while (true) {
-        children.add(
+  @override
+  List<DiagnosticsNode> debugDescribeChildren() {
+    final List<DiagnosticsNode> offstageChildren = <DiagnosticsNode>[];
+    final List<DiagnosticsNode> onstageChildren = <DiagnosticsNode>[];
+
+    int count = 1;
+    bool onstage = false;
+    RenderBox child = firstChild;
+    final RenderBox firstOnstageChild = _firstOnstageChild;
+    while (child != null) {
+      if (child == firstOnstageChild) {
+        onstage = true;
+        count = 1;
+      }
+
+      if (onstage) {
+        onstageChildren.add(
+          child.toDiagnosticsNode(
+            name: 'onstage $count',
+          ),
+        );
+      } else {
+        offstageChildren.add(
           child.toDiagnosticsNode(
             name: 'offstage $count',
             style: DiagnosticsTreeStyle.offstage,
           ),
         );
-        if (child == lastChild)
-          break;
-        final StackParentData childParentData = child.parentData as StackParentData;
-        child = childParentData.nextSibling;
-        count += 1;
       }
-    } else {
-      children.add(
+
+      final StackParentData childParentData = child.parentData as StackParentData;
+      child = childParentData.nextSibling;
+      count += 1;
+    }
+
+    return <DiagnosticsNode>[
+      ...onstageChildren,
+      if (offstageChildren.isNotEmpty)
+        ...offstageChildren
+      else
         DiagnosticsNode.message(
           'no offstage children',
           style: DiagnosticsTreeStyle.offstage,
         ),
-      );
-    }
-    return children;
-  }
-
-  @override
-  void visitChildrenForSemantics(RenderObjectVisitor visitor) {
-    if (child != null)
-      visitor(child);
+    ];
   }
 }

packages/flutter/test/cupertino/nav_bar_transition_test.dart

@@ -72,12 +72,9 @@ CupertinoPageScaffold scaffoldForNavBar(Widget navBar) {
 }
 
 Finder flying(WidgetTester tester, Finder finder) {
-  final RenderObjectWithChildMixin<RenderStack> theater =
-      tester.renderObject(find.byType(Overlay));
-  final RenderStack theaterStack = theater.child;
+  final ContainerRenderObjectMixin<RenderBox, StackParentData> theater = tester.renderObject(find.byType(Overlay));
   final Finder lastOverlayFinder = find.byElementPredicate((Element element) {
-    return element is RenderObjectElement &&
-        element.renderObject == theaterStack.lastChild;
+    return element is RenderObjectElement && element.renderObject == theater.lastChild;
   });
 
   assert(

packages/flutter/test/material/debug_test.dart

@@ -132,8 +132,8 @@ void main() {
       '     Offstage\n'
       '     _ModalScopeStatus\n'
       '     _ModalScope<dynamic>-[LabeledGlobalKey<_ModalScopeState<dynamic>>#969b7]\n'
-      '     _OverlayEntry-[LabeledGlobalKey<_OverlayEntryState>#7a3ae]\n'
-      '     Stack\n'
+      '     TickerMode\n'
+      '     _OverlayEntryWidget-[LabeledGlobalKey<_OverlayEntryWidgetState>#545d0]\n'
       '     _Theatre\n'
       '     Overlay-[LabeledGlobalKey<OverlayState>#31a52]\n'
       '     _FocusMarker\n'

packages/flutter/test/material/stepper_test.dart

@@ -524,23 +524,27 @@ void main() {
     expect(errorDetails, isNotNull);
     expect(errorDetails.stack, isNotNull);
     // Check the ErrorDetails without the stack trace
-    final List<String> lines =  errorDetails.toString().split('\n');
+    final String fullErrorMessage = errorDetails.toString();
+    final List<String> lines = fullErrorMessage.split('\n');
     // The lines in the middle of the error message contain the stack trace
     // which will change depending on where the test is run.
-    expect(lines.length, greaterThan(7));
-    expect(
-      lines.take(8).join('\n'),
-      equalsIgnoringHashCodes(
-        '══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞════════════════════════\n'
-        'The following assertion was thrown building Stepper(dirty,\n'
-        'dependencies: [_LocalizationsScope-[GlobalKey#00000]], state:\n'
-        '_StepperState#00000):\n'
-        'Steppers must not be nested.\n'
-        'The material specification advises that one should avoid\n'
-        'embedding steppers within steppers.\n'
-        'https://material.io/archive/guidelines/components/steppers.html#steppers-usage'
-      ),
-    );
+    final String errorMessage = lines.takeWhile(
+      (String line) => line != '',
+    ).join('\n');
+    expect(errorMessage.length, lessThan(fullErrorMessage.length));
+    expect(errorMessage, startsWith(
+      '══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞════════════════════════\n'
+      'The following assertion was thrown building Stepper('
+    ));
+    // The description string of the stepper looks slightly different depending
+    // on the platform and is omitted here.
+    expect(errorMessage, endsWith(
+      '):\n'
+      'Steppers must not be nested.\n'
+      'The material specification advises that one should avoid\n'
+      'embedding steppers within steppers.\n'
+      'https://material.io/archive/guidelines/components/steppers.html#steppers-usage'
+    ));
   });
 
   ///https://github.com/flutter/flutter/issues/16920

packages/flutter/test/widgets/navigator_test.dart

@@ -1465,6 +1465,33 @@ void main() {
     expect(find.byKey(const ValueKey<String>('/A/B')), findsNothing); // popped
     expect(find.byKey(const ValueKey<String>('/C')), findsOneWidget);
   });
+
+  testWidgets('Pushing opaque Route does not rebuild routes below', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/45797.
+
+    final GlobalKey<NavigatorState> navigator = GlobalKey<NavigatorState>();
+    final Key bottomRoute = UniqueKey();
+    final Key topRoute = UniqueKey();
+    await tester.pumpWidget(
+      MaterialApp(
+        navigatorKey: navigator,
+        routes: <String, WidgetBuilder>{
+          '/' : (BuildContext context) => StatefulTestWidget(key: bottomRoute),
+          '/a': (BuildContext context) => StatefulTestWidget(key: topRoute),
+        },
+      ),
+    );
+    expect(tester.state<StatefulTestState>(find.byKey(bottomRoute)).rebuildCount, 1);
+
+    navigator.currentState.pushNamed('/a');
+    await tester.pumpAndSettle();
+
+    // Bottom route is offstage and did not rebuild.
+    expect(find.byKey(bottomRoute), findsNothing);
+    expect(tester.state<StatefulTestState>(find.byKey(bottomRoute, skipOffstage: false)).rebuildCount, 1);
+
+    expect(tester.state<StatefulTestState>(find.byKey(topRoute)).rebuildCount, 1);
+  });
 }
 
 class NoAnimationPageRoute extends PageRouteBuilder<void> {
@@ -1478,3 +1505,20 @@ class NoAnimationPageRoute extends PageRouteBuilder<void> {
     return super.createAnimationController()..value = 1.0;
   }
 }
+
+class StatefulTestWidget extends StatefulWidget {
+  const StatefulTestWidget({Key key}) : super(key: key);
+
+  @override
+  State<StatefulTestWidget> createState() => StatefulTestState();
+}
+
+class StatefulTestState extends State<StatefulTestWidget> {
+  int rebuildCount = 0;
+
+  @override
+  Widget build(BuildContext context) {
+    rebuildCount += 1;
+    return Container();
+  }
+}

packages/flutter/test/widgets/overlay_test.dart

@@ -6,6 +6,8 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
+import 'semantics_tester.dart';
+
 void main() {
   testWidgets('OverflowEntries context contains Overlay', (WidgetTester tester) async {
     final GlobalKey overlayKey = GlobalKey();
@@ -25,6 +27,9 @@ void main() {
                 return Container();
               },
             ),
+            OverlayEntry(
+              builder: (BuildContext context) => Container(),
+            )
           ],
         ),
       ),
@@ -36,36 +41,42 @@ void main() {
     expect(
       theater.toStringDeep(minLevel: DiagnosticLevel.info),
       equalsIgnoringHashCodes(
-        '_RenderTheatre#f5cf2\n'
-            ' │ parentData: <none>\n'
-            ' │ constraints: BoxConstraints(w=800.0, h=600.0)\n'
-            ' │ size: Size(800.0, 600.0)\n'
-            ' │\n'
-            ' ├─onstage: RenderStack#39819\n'
-            ' ╎ │ parentData: not positioned; offset=Offset(0.0, 0.0) (can use\n'
-            ' ╎ │   size)\n'
-            ' ╎ │ constraints: BoxConstraints(w=800.0, h=600.0)\n'
-            ' ╎ │ size: Size(800.0, 600.0)\n'
-            ' ╎ │ alignment: AlignmentDirectional.topStart\n'
-            ' ╎ │ textDirection: ltr\n'
-            ' ╎ │ fit: expand\n'
-            ' ╎ │ overflow: clip\n'
-            ' ╎ │\n'
-            ' ╎ └─child 1: RenderLimitedBox#d1448\n'
-            ' ╎   │ parentData: not positioned; offset=Offset(0.0, 0.0) (can use\n'
-            ' ╎   │   size)\n'
-            ' ╎   │ constraints: BoxConstraints(w=800.0, h=600.0)\n'
-            ' ╎   │ size: Size(800.0, 600.0)\n'
-            ' ╎   │ maxWidth: 0.0\n'
-            ' ╎   │ maxHeight: 0.0\n'
-            ' ╎   │\n'
-            ' ╎   └─child: RenderConstrainedBox#e8b87\n'
-            ' ╎       parentData: <none> (can use size)\n'
-            ' ╎       constraints: BoxConstraints(w=800.0, h=600.0)\n'
-            ' ╎       size: Size(800.0, 600.0)\n'
-            ' ╎       additionalConstraints: BoxConstraints(biggest)\n'
-            ' ╎\n'
-            ' └╌no offstage children\n'
+        '_RenderTheatre#744c9\n'
+        ' │ parentData: <none>\n'
+        ' │ constraints: BoxConstraints(w=800.0, h=600.0)\n'
+        ' │ size: Size(800.0, 600.0)\n'
+        ' │ skipCount: 0\n'
+        ' │ textDirection: ltr\n'
+        ' │\n'
+        ' ├─onstage 1: RenderLimitedBox#bb803\n'
+        ' │ │ parentData: not positioned; offset=Offset(0.0, 0.0) (can use\n'
+        ' │ │   size)\n'
+        ' │ │ constraints: BoxConstraints(w=800.0, h=600.0)\n'
+        ' │ │ size: Size(800.0, 600.0)\n'
+        ' │ │ maxWidth: 0.0\n'
+        ' │ │ maxHeight: 0.0\n'
+        ' │ │\n'
+        ' │ └─child: RenderConstrainedBox#62707\n'
+        ' │     parentData: <none> (can use size)\n'
+        ' │     constraints: BoxConstraints(w=800.0, h=600.0)\n'
+        ' │     size: Size(800.0, 600.0)\n'
+        ' │     additionalConstraints: BoxConstraints(biggest)\n'
+        ' │\n'
+        ' ├─onstage 2: RenderLimitedBox#af5f1\n'
+        ' ╎ │ parentData: not positioned; offset=Offset(0.0, 0.0) (can use\n'
+        ' ╎ │   size)\n'
+        ' ╎ │ constraints: BoxConstraints(w=800.0, h=600.0)\n'
+        ' ╎ │ size: Size(800.0, 600.0)\n'
+        ' ╎ │ maxWidth: 0.0\n'
+        ' ╎ │ maxHeight: 0.0\n'
+        ' ╎ │\n'
+        ' ╎ └─child: RenderConstrainedBox#69c48\n'
+        ' ╎     parentData: <none> (can use size)\n'
+        ' ╎     constraints: BoxConstraints(w=800.0, h=600.0)\n'
+        ' ╎     size: Size(800.0, 600.0)\n'
+        ' ╎     additionalConstraints: BoxConstraints(biggest)\n'
+        ' ╎\n'
+        ' └╌no offstage children\n'
       ),
     );
   });
@@ -103,60 +114,52 @@ void main() {
     expect(
       theater.toStringDeep(minLevel: DiagnosticLevel.info),
       equalsIgnoringHashCodes(
-        '_RenderTheatre#b22a8\n'
+        '_RenderTheatre#385b3\n'
         ' │ parentData: <none>\n'
         ' │ constraints: BoxConstraints(w=800.0, h=600.0)\n'
         ' │ size: Size(800.0, 600.0)\n'
+        ' │ skipCount: 2\n'
+        ' │ textDirection: ltr\n'
         ' │\n'
-        ' ├─onstage: RenderStack#eab87\n'
+        ' ├─onstage 1: RenderLimitedBox#0a77a\n'
         ' ╎ │ parentData: not positioned; offset=Offset(0.0, 0.0) (can use\n'
         ' ╎ │   size)\n'
         ' ╎ │ constraints: BoxConstraints(w=800.0, h=600.0)\n'
         ' ╎ │ size: Size(800.0, 600.0)\n'
-        ' ╎ │ alignment: AlignmentDirectional.topStart\n'
-        ' ╎ │ textDirection: ltr\n'
-        ' ╎ │ fit: expand\n'
-        ' ╎ │ overflow: clip\n'
+        ' ╎ │ maxWidth: 0.0\n'
+        ' ╎ │ maxHeight: 0.0\n'
         ' ╎ │\n'
-        ' ╎ └─child 1: RenderLimitedBox#ca15b\n'
-        ' ╎   │ parentData: not positioned; offset=Offset(0.0, 0.0) (can use\n'
-        ' ╎   │   size)\n'
-        ' ╎   │ constraints: BoxConstraints(w=800.0, h=600.0)\n'
-        ' ╎   │ size: Size(800.0, 600.0)\n'
-        ' ╎   │ maxWidth: 0.0\n'
-        ' ╎   │ maxHeight: 0.0\n'
-        ' ╎   │\n'
-        ' ╎   └─child: RenderConstrainedBox#dffe5\n'
-        ' ╎       parentData: <none> (can use size)\n'
-        ' ╎       constraints: BoxConstraints(w=800.0, h=600.0)\n'
-        ' ╎       size: Size(800.0, 600.0)\n'
-        ' ╎       additionalConstraints: BoxConstraints(biggest)\n'
+        ' ╎ └─child: RenderConstrainedBox#21f3a\n'
+        ' ╎     parentData: <none> (can use size)\n'
+        ' ╎     constraints: BoxConstraints(w=800.0, h=600.0)\n'
+        ' ╎     size: Size(800.0, 600.0)\n'
+        ' ╎     additionalConstraints: BoxConstraints(biggest)\n'
         ' ╎\n'
-        ' ╎╌offstage 1: RenderLimitedBox#b6f09 NEEDS-LAYOUT NEEDS-PAINT\n'
+        ' ╎╌offstage 1: RenderLimitedBox#62c8c NEEDS-LAYOUT NEEDS-PAINT\n'
         ' ╎ │ parentData: not positioned; offset=Offset(0.0, 0.0)\n'
         ' ╎ │ constraints: MISSING\n'
         ' ╎ │ size: MISSING\n'
         ' ╎ │ maxWidth: 0.0\n'
         ' ╎ │ maxHeight: 0.0\n'
         ' ╎ │\n'
-        ' ╎ └─child: RenderConstrainedBox#5a057 NEEDS-LAYOUT NEEDS-PAINT\n'
+        ' ╎ └─child: RenderConstrainedBox#425fa NEEDS-LAYOUT NEEDS-PAINT\n'
         ' ╎     parentData: <none>\n'
         ' ╎     constraints: MISSING\n'
         ' ╎     size: MISSING\n'
         ' ╎     additionalConstraints: BoxConstraints(biggest)\n'
         ' ╎\n'
-        ' └╌offstage 2: RenderLimitedBox#f689e NEEDS-LAYOUT NEEDS-PAINT\n'
+        ' └╌offstage 2: RenderLimitedBox#03ae2 NEEDS-LAYOUT NEEDS-PAINT\n'
         '   │ parentData: not positioned; offset=Offset(0.0, 0.0)\n'
         '   │ constraints: MISSING\n'
         '   │ size: MISSING\n'
         '   │ maxWidth: 0.0\n'
         '   │ maxHeight: 0.0\n'
         '   │\n'
-        '   └─child: RenderConstrainedBox#c15f0 NEEDS-LAYOUT NEEDS-PAINT\n'
+        '   └─child: RenderConstrainedBox#b4d48 NEEDS-LAYOUT NEEDS-PAINT\n'
         '       parentData: <none>\n'
         '       constraints: MISSING\n'
         '       size: MISSING\n'
-        '       additionalConstraints: BoxConstraints(biggest)\n'
+        '       additionalConstraints: BoxConstraints(biggest)\n',
       ),
     );
   });
@@ -698,4 +701,261 @@ void main() {
     expect(find.byKey(root), findsNothing);
     expect(find.byKey(top), findsOneWidget);
   });
+
+  testWidgets('OverlayEntries do not rebuild when opaqueness changes', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/45797.
+
+    final GlobalKey<OverlayState> overlayKey = GlobalKey<OverlayState>();
+    final Key bottom = UniqueKey();
+    final Key middle = UniqueKey();
+    final Key top = UniqueKey();
+    final Widget bottomWidget = StatefulTestWidget(key: bottom);
+    final Widget middleWidget = StatefulTestWidget(key: middle);
+    final Widget topWidget = StatefulTestWidget(key: top);
+
+    final OverlayEntry bottomEntry = OverlayEntry(
+      maintainState: true,
+      builder: (BuildContext context) {
+        return bottomWidget;
+      },
+    );
+    final OverlayEntry middleEntry = OverlayEntry(
+      maintainState: true,
+      builder: (BuildContext context) {
+        return middleWidget;
+      },
+    );
+    final OverlayEntry topEntry = OverlayEntry(
+      maintainState: true,
+      builder: (BuildContext context) {
+        return topWidget;
+      },
+    );
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Overlay(
+          key: overlayKey,
+          initialEntries: <OverlayEntry>[
+            bottomEntry,
+            middleEntry,
+            topEntry,
+          ],
+        ),
+      ),
+    );
+
+    // All widgets are onstage.
+    expect(tester.state<StatefulTestState>(find.byKey(bottom)).rebuildCount, 1);
+    expect(tester.state<StatefulTestState>(find.byKey(middle)).rebuildCount, 1);
+    expect(tester.state<StatefulTestState>(find.byKey(top)).rebuildCount, 1);
+
+    middleEntry.opaque = true;
+    await tester.pump();
+
+    // Bottom widget is offstage and did not rebuild.
+    expect(find.byKey(bottom), findsNothing);
+    expect(tester.state<StatefulTestState>(find.byKey(bottom, skipOffstage: false)).rebuildCount, 1);
+    expect(tester.state<StatefulTestState>(find.byKey(middle)).rebuildCount, 1);
+    expect(tester.state<StatefulTestState>(find.byKey(top)).rebuildCount, 1);
+  });
+
+  testWidgets('OverlayEntries do not rebuild when opaque entry is added', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/45797.
+
+    final GlobalKey<OverlayState> overlayKey = GlobalKey<OverlayState>();
+    final Key bottom = UniqueKey();
+    final Key middle = UniqueKey();
+    final Key top = UniqueKey();
+    final Widget bottomWidget = StatefulTestWidget(key: bottom);
+    final Widget middleWidget = StatefulTestWidget(key: middle);
+    final Widget topWidget = StatefulTestWidget(key: top);
+
+    final OverlayEntry bottomEntry = OverlayEntry(
+      maintainState: true,
+      builder: (BuildContext context) {
+        return bottomWidget;
+      },
+    );
+    final OverlayEntry middleEntry = OverlayEntry(
+      opaque: true,
+      maintainState: true,
+      builder: (BuildContext context) {
+        return middleWidget;
+      },
+    );
+    final OverlayEntry topEntry = OverlayEntry(
+      maintainState: true,
+      builder: (BuildContext context) {
+        return topWidget;
+      },
+    );
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Overlay(
+          key: overlayKey,
+          initialEntries: <OverlayEntry>[
+            bottomEntry,
+            topEntry,
+          ],
+        ),
+      ),
+    );
+
+    // Both widgets are onstage.
+    expect(tester.state<StatefulTestState>(find.byKey(bottom)).rebuildCount, 1);
+    expect(tester.state<StatefulTestState>(find.byKey(top)).rebuildCount, 1);
+
+    overlayKey.currentState.rearrange(<OverlayEntry>[
+      bottomEntry, middleEntry, topEntry,
+    ]);
+    await tester.pump();
+
+    // Bottom widget is offstage and did not rebuild.
+    expect(find.byKey(bottom), findsNothing);
+    expect(tester.state<StatefulTestState>(find.byKey(bottom, skipOffstage: false)).rebuildCount, 1);
+    expect(tester.state<StatefulTestState>(find.byKey(middle)).rebuildCount, 1);
+    expect(tester.state<StatefulTestState>(find.byKey(top)).rebuildCount, 1);
+  });
+
+  testWidgets('entries below opaque entries are ignored for hit testing', (WidgetTester tester) async {
+    final GlobalKey<OverlayState> overlayKey = GlobalKey<OverlayState>();
+    int bottomTapCount = 0;
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Overlay(
+          key: overlayKey,
+          initialEntries: <OverlayEntry>[
+            OverlayEntry(
+              maintainState: true,
+              builder: (BuildContext context) {
+                return GestureDetector(
+                  onTap: () {
+                    bottomTapCount++;
+                  },
+                );
+              },
+            ),
+          ],
+        ),
+      ),
+    );
+
+    expect(bottomTapCount, 0);
+    await tester.tap(find.byKey(overlayKey));
+    expect(bottomTapCount, 1);
+
+    overlayKey.currentState.insert(OverlayEntry(
+      maintainState: true,
+      opaque: true,
+      builder: (BuildContext context) {
+        return Container();
+      },
+    ));
+    await tester.pump();
+
+    // Bottom is offstage and does not receive tap events.
+    expect(find.byType(GestureDetector), findsNothing);
+    expect(find.byType(GestureDetector, skipOffstage: false), findsOneWidget);
+    await tester.tap(find.byKey(overlayKey));
+    expect(bottomTapCount, 1);
+
+    int topTapCount = 0;
+    overlayKey.currentState.insert(OverlayEntry(
+      maintainState: true,
+      opaque: true,
+      builder: (BuildContext context) {
+        return GestureDetector(
+          onTap: () {
+            topTapCount++;
+          },
+        );
+      },
+    ));
+    await tester.pump();
+
+    expect(topTapCount, 0);
+    await tester.tap(find.byKey(overlayKey));
+    expect(topTapCount, 1);
+    expect(bottomTapCount, 1);
+  });
+
+  testWidgets('Semantics of entries below opaque entries are ignored', (WidgetTester tester) async {
+    final SemanticsTester semantics = SemanticsTester(tester);
+    final GlobalKey<OverlayState> overlayKey = GlobalKey<OverlayState>();
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Overlay(
+          key: overlayKey,
+          initialEntries: <OverlayEntry>[
+            OverlayEntry(
+              maintainState: true,
+              builder: (BuildContext context) {
+                return const Text('bottom');
+              },
+            ),
+            OverlayEntry(
+              maintainState: true,
+              opaque: true,
+              builder: (BuildContext context) {
+                return const Text('top');
+              },
+            ),
+          ],
+        ),
+      ),
+    );
+    expect(find.text('bottom'), findsNothing);
+    expect(find.text('bottom', skipOffstage: false), findsOneWidget);
+    expect(find.text('top'), findsOneWidget);
+    expect(semantics, includesNodeWith(label: 'top'));
+    expect(semantics, isNot(includesNodeWith(label: 'bottom')));
+
+    semantics.dispose();
+  });
+
+  testWidgets('Can used Positioned within OverlayEntry', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Overlay(
+          initialEntries: <OverlayEntry>[
+            OverlayEntry(
+              builder: (BuildContext context) {
+                return const Positioned(
+                  left: 145,
+                  top: 123,
+                  child: Text('positioned child'),
+                );
+              },
+            ),
+          ],
+        ),
+      ),
+    );
+
+    expect(tester.getTopLeft(find.text('positioned child')), const Offset(145, 123));
+  });
+}
+
+class StatefulTestWidget extends StatefulWidget {
+  const StatefulTestWidget({Key key}) : super(key: key);
+
+  @override
+  State<StatefulTestWidget> createState() => StatefulTestState();
+}
+
+class StatefulTestState extends State<StatefulTestWidget> {
+  int rebuildCount = 0;
+
+  @override
+  Widget build(BuildContext context) {
+    rebuildCount += 1;
+    return Container();
+  }
 }