Do not rebuild Routes when a new opaque Route is pushed on top (#48900)

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/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

@@ -529,12 +529,13 @@ void main() {
     // which will change depending on where the test is run.
     expect(lines.length, greaterThan(7));
     expect(
-      lines.take(8).join('\n'),
+      lines.take(9).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'
+        'dependencies: [TickerMode,\n'
+        '_LocalizationsScope-[GlobalKey#6b31b]], state:\n'
+        '_StepperState#1bf00):\n'
         'Steppers must not be nested.\n'
         'The material specification advises that one should avoid\n'
         'embedding steppers within steppers.\n'

packages/flutter/test/widgets/navigator_test.dart

@@ -1186,6 +1186,33 @@ 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> {
@@ -1199,3 +1226,20 @@ 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();
+  }
+}