Reland "Fix RefreshIndicator performance issue"

packages/flutter/lib/src/material/refresh_indicator.dart

@@ -409,34 +409,34 @@ class RefreshIndicatorState extends State<RefreshIndicator> with TickerProviderS
     return _pendingRefreshFuture;
   }
 
-  final GlobalKey _key = GlobalKey();
-
   @override
   Widget build(BuildContext context) {
     assert(debugCheckHasMaterialLocalizations(context));
     final Widget child = NotificationListener<ScrollNotification>(
-      key: _key,
       onNotification: _handleScrollNotification,
       child: NotificationListener<OverscrollIndicatorNotification>(
         onNotification: _handleGlowNotification,
         child: widget.child,
       ),
     );
-    if (_mode == null) {
-      assert(_dragOffset == null);
-      assert(_isIndicatorAtTop == null);
-      return child;
-    }
-    assert(_dragOffset != null);
-    assert(_isIndicatorAtTop != null);
+    assert(() {
+      if (_mode == null) {
+        assert(_dragOffset == null);
+        assert(_isIndicatorAtTop == null);
+      } else {
+        assert(_dragOffset != null);
+        assert(_isIndicatorAtTop != null);
+      }
+      return true;
+    }());
 
     final bool showIndeterminateIndicator =
       _mode == _RefreshIndicatorMode.refresh || _mode == _RefreshIndicatorMode.done;
 
     return Stack(
       children: <Widget>[
         child,
-        Positioned(
+        if (_mode != null) Positioned(
           top: _isIndicatorAtTop ? 0.0 : null,
           bottom: !_isIndicatorAtTop ? 0.0 : null,
           left: 0.0,

packages/flutter/test/material/refresh_indicator_test.dart

@@ -420,4 +420,39 @@ void main() {
     expect(controller.offset, lessThan(0.0));
     expect(refreshCalled, isTrue);
   }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS,  TargetPlatform.macOS }));
-}
+
+  testWidgets('RefreshIndicator does not force child to relayout', (WidgetTester tester) async {
+    int layoutCount = 0;
+
+    Widget layoutCallback(BuildContext context, BoxConstraints constraints) {
+      layoutCount++;
+      return ListView(
+        physics: const AlwaysScrollableScrollPhysics(),
+        children: <String>['A', 'B', 'C', 'D', 'E', 'F'].map<Widget>((String item) {
+          return SizedBox(
+            height: 200.0,
+            child: Text(item),
+          );
+        }).toList(),
+      );
+    }
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: RefreshIndicator(
+          onRefresh: refresh,
+          child: LayoutBuilder(builder: layoutCallback),
+        ),
+      ),
+    );
+
+    await tester.fling(find.text('A'), const Offset(0.0, 300.0), 1000.0); // trigger refresh
+    await tester.pump();
+
+    await tester.pump(const Duration(seconds: 1)); // finish the scroll animation
+    await tester.pump(const Duration(seconds: 1)); // finish the indicator settle animation
+    await tester.pump(const Duration(seconds: 1)); // finish the indicator hide animation
+
+    expect(layoutCount, 1);
+  });
+}