Handle ListView item size changes that cause underflow (#9586)

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

@@ -97,6 +97,7 @@ class RenderSliverList extends RenderSliverMultiBoxAdaptor {
          earliestScrollOffset = childScrollOffset(earliestUsefulChild)) {
       // We have to add children before the earliestUsefulChild.
       earliestUsefulChild = insertAndLayoutLeadingChild(childConstraints, parentUsesSize: true);
+
       if (earliestUsefulChild == null) {
         // We ran out of children before reaching the scroll offset.
         // We must inform our parent that this sliver cannot fulfill
@@ -108,8 +109,33 @@ class RenderSliverList extends RenderSliverMultiBoxAdaptor {
         childParentData.layoutOffset = 0.0;
         return;
       }
+
+      final double firstChildScrollOffset = earliestScrollOffset - paintExtentOf(firstChild);
+      if (firstChildScrollOffset < 0.0) {
+        // The first child doesn't fit within the viewport (underflow) and
+        // there may be additional children above it. Find the real first child
+        // and then correct the scroll position so that there's room for all and
+        // so that the trailing edge of the original firstChild appears where it
+        // was before the scroll offset correction.
+        // TODO(hansmuller): do this work incrementally, instead of all at once,
+        // i.e. find a way to avoid visiting ALL of the children whose offset
+        // is < 0 before returning for the scroll correction.
+        double correction = 0.0;
+        while (earliestUsefulChild != null) {
+          assert(firstChild == earliestUsefulChild);
+          correction += paintExtentOf(firstChild);
+          earliestUsefulChild = insertAndLayoutLeadingChild(childConstraints, parentUsesSize: true);
+        }
+        geometry = new SliverGeometry(
+          scrollOffsetCorrection: correction - earliestScrollOffset,
+        );
+        final SliverMultiBoxAdaptorParentData childParentData = firstChild.parentData;
+        childParentData.layoutOffset = 0.0;
+        return;
+      }
+
       final SliverMultiBoxAdaptorParentData childParentData = earliestUsefulChild.parentData;
-      childParentData.layoutOffset = earliestScrollOffset - paintExtentOf(firstChild);
+      childParentData.layoutOffset = firstChildScrollOffset;
       assert(earliestUsefulChild == firstChild);
       leadingChildWithLayout = earliestUsefulChild;
       trailingChildWithLayout ??= earliestUsefulChild;

packages/flutter/test/widgets/list_view_correction_test.dart

@@ -53,4 +53,82 @@ void main() {
     expect(tester.getTopLeft(find.text('2')).dy, equals(200.0));
   });
 
+  testWidgets('ListView can handle inserts at 0', (WidgetTester tester) async {
+    final ScrollController controller = new ScrollController();
+    await tester.pumpWidget(new ListView(
+      controller: controller,
+      children: <Widget>[
+        new Container(height: 400.0, child: const Text('0')),
+        new Container(height: 400.0, child: const Text('1')),
+        new Container(height: 400.0, child: const Text('2')),
+        new Container(height: 400.0, child: const Text('3')),
+      ],
+    ));
+    expect(find.text('0'), findsOneWidget);
+    expect(find.text('1'), findsOneWidget);
+    expect(find.text('2'), findsNothing);
+    expect(find.text('3'), findsNothing);
+
+    final Finder findItemA = find.descendant(of: find.byType(Container), matching: find.text('A'));
+    final Finder findItemB = find.descendant(of: find.byType(Container), matching: find.text('B'));
+    await tester.pumpWidget(new ListView(
+      controller: controller,
+      children: <Widget>[
+        new Container(height: 10.0, child: const Text('A')),
+        new Container(height: 10.0, child: const Text('B')),
+        new Container(height: 400.0, child: const Text('0')),
+        new Container(height: 400.0, child: const Text('1')),
+        new Container(height: 400.0, child: const Text('2')),
+        new Container(height: 400.0, child: const Text('3')),
+      ],
+    ));
+    expect(find.text('A'), findsOneWidget);
+    expect(find.text('B'), findsOneWidget);
+    expect(tester.getTopLeft(findItemA).dy, 0.0);
+    expect(tester.getBottomRight(findItemA).dy, 10.0);
+    expect(tester.getTopLeft(findItemB).dy, 10.0);
+    expect(tester.getBottomRight(findItemB).dy, 20.0);
+
+
+    controller.jumpTo(1200.0);
+    await tester.pump();
+    expect(find.text('A'), findsNothing);
+    expect(find.text('B'), findsNothing);
+
+    await tester.pumpWidget(new ListView(
+      controller: controller,
+      children: <Widget>[
+        new Container(height: 200.0, child: const Text('A')),
+        new Container(height: 200.0, child: const Text('B')),
+        new Container(height: 400.0, child: const Text('0')),
+        new Container(height: 400.0, child: const Text('1')),
+        new Container(height: 400.0, child: const Text('2')),
+        new Container(height: 400.0, child: const Text('3')),
+      ],
+    ));
+    expect(find.text('A'), findsNothing);
+    expect(find.text('B'), findsNothing);
+
+    // Scrolling to 0 causes items A and B to underflow (extend below
+    // scrollOffset 0) because their heights have grown from 10 - 200.
+    // RenderSliver list corrects the scroll offset in this case. Only item
+    // B will become visible and item B's bottom edge will still appear
+    // where it was when its height was 10.0.
+    controller.jumpTo(0.0);
+    await tester.pump();
+    expect(find.text('B'), findsOneWidget);
+    expect(controller.offset, greaterThan(0.0)); // RenderSliverList corrected the offset.
+    expect(tester.getTopLeft(findItemB).dy, -180.0);
+    expect(tester.getBottomRight(findItemB).dy, 20.0);
+
+
+    controller.jumpTo(0.0);
+    await tester.pump();
+    expect(find.text('A'), findsOneWidget);
+    expect(find.text('B'), findsOneWidget);
+    expect(tester.getTopLeft(findItemA).dy, 0.0);
+    expect(tester.getBottomRight(findItemA).dy, 200.0);
+    expect(tester.getTopLeft(findItemB).dy, 200.0);
+    expect(tester.getBottomRight(findItemB).dy, 400.0);
+  });
 }