Performance Improvement for SliverFillRemaining (#44471)

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

@@ -157,16 +157,15 @@ class RenderSliverFillRemaining extends RenderSliverSingleBoxAdapter {
           parentUsesSize: true,
         );
     } else if (child != null) {
-      child.layout(constraints.asBoxConstraints(), parentUsesSize: true);
-
       switch (constraints.axis) {
         case Axis.horizontal:
-          childExtent = child.size.width;
+          childExtent = child.getMaxIntrinsicWidth(constraints.crossAxisExtent);
           break;
         case Axis.vertical:
-          childExtent = child.size.height;
+          childExtent = child.getMaxIntrinsicHeight(constraints.crossAxisExtent);
           break;
       }
+
       if (constraints.precedingScrollExtent > constraints.viewportMainAxisExtent || childExtent > extent)
         extent = childExtent;
       if (maxExtent < extent)
@@ -179,6 +178,8 @@ class RenderSliverFillRemaining extends RenderSliverSingleBoxAdapter {
           ),
           parentUsesSize: true,
         );
+      } else {
+        child.layout(constraints.asBoxConstraints(), parentUsesSize: true);
       }
     }
 

packages/flutter/test/widgets/sliver_fill_remaining_test.dart

@@ -70,12 +70,18 @@ void main() {
       child: Container(
         color: Colors.amber,
         height: 150.0,
+        width: 150,
       ),
     );
-    Widget boilerplate(List<Widget> slivers, {ScrollController controller}) {
+    Widget boilerplate(
+      List<Widget> slivers, {
+      ScrollController controller,
+      Axis scrollDirection = Axis.vertical,
+    }) {
       return MaterialApp(
         home: Scaffold(
           body: CustomScrollView(
+            scrollDirection: scrollDirection,
             slivers: slivers,
             controller: controller,
           ),
@@ -128,8 +134,12 @@ void main() {
         ),
       ];
       await tester.pumpWidget(boilerplate(slivers));
-      final RenderBox box = tester.renderObject<RenderBox>(find.byType(Container).last);
+      RenderBox box = tester.renderObject<RenderBox>(find.byType(Container).last);
       expect(box.size.height, equals(450));
+
+      await tester.pumpWidget(boilerplate(slivers, scrollDirection: Axis.horizontal));
+      box = tester.renderObject<RenderBox>(find.byType(Container).last);
+      expect(box.size.width, equals(650));
     });
 
     testWidgets('child with size is sized by extent when false', (WidgetTester tester) async {
@@ -158,6 +168,9 @@ void main() {
       final Finder button = find.byType(RaisedButton);
       expect(tester.getBottomLeft(button).dy, equals(600.0));
       expect(tester.getCenter(button).dx, equals(400.0));
+
+      await tester.pumpWidget(boilerplate(slivers, scrollDirection: Axis.horizontal));
+      expect(tester.renderObject<RenderBox>(find.byKey(key)).size.width, equals(650));
     });
 
     testWidgets('extent is overridden by child with larger size when false', (WidgetTester tester) async {
@@ -168,12 +181,17 @@ void main() {
           child: Container(
             color: Colors.blue,
             height: 600,
+            width: 1000,
           ),
         ),
       ];
       await tester.pumpWidget(boilerplate(slivers));
-      final RenderBox box = tester.renderObject<RenderBox>(find.byType(Container).last);
+      RenderBox box = tester.renderObject<RenderBox>(find.byType(Container).last);
       expect(box.size.height, equals(600));
+
+      await tester.pumpWidget(boilerplate(slivers, scrollDirection: Axis.horizontal));
+      box = tester.renderObject<RenderBox>(find.byType(Container).last);
+      expect(box.size.width, equals(1000));
     });
 
     testWidgets('extent is overridden by child size if precedingScrollExtent > viewportMainAxisExtent when false', (WidgetTester tester) async {
@@ -313,7 +331,6 @@ void main() {
       final Finder button = find.byType(RaisedButton);
       expect(tester.getBottomLeft(button).dy, equals(550.0));
       expect(tester.getCenter(button).dx, equals(400.0));
-      debugDefaultTargetPlatformOverride = null;
 
       // Drag for overscroll
       await tester.drag(find.byType(Scrollable), const Offset(0.0, -50.0));
@@ -418,7 +435,6 @@ void main() {
       final Finder button = find.byType(RaisedButton);
       expect(tester.getBottomLeft(button).dy, equals(550.0));
       expect(tester.getCenter(button).dx, equals(400.0));
-      debugDefaultTargetPlatformOverride = null;
 
       await tester.drag(find.byType(Scrollable), const Offset(0.0, -50.0));
       await tester.pump();