Fix clipping for SliverLists (#18410)

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

@@ -248,13 +248,16 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
       to: trailingScrollOffset,
     );
 
+    final double targetEndScrollOffsetForPaint = constraints.scrollOffset + constraints.remainingPaintExtent;
+    final int targetLastIndexForPaint = targetEndScrollOffsetForPaint.isFinite ?
+        getMaxChildIndexForScrollOffset(targetEndScrollOffsetForPaint, itemExtent) : null;
     geometry = new SliverGeometry(
       scrollExtent: estimatedMaxScrollOffset,
       paintExtent: paintExtent,
       cacheExtent: cacheExtent,
       maxPaintExtent: estimatedMaxScrollOffset,
       // Conservative to avoid flickering away the clip during scroll.
-      hasVisualOverflow: (targetLastIndex != null && lastIndex >= targetLastIndex)
+      hasVisualOverflow: (targetLastIndexForPaint != null && lastIndex >= targetLastIndexForPaint)
         || constraints.scrollOffset > 0.0,
     );
 

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

@@ -274,13 +274,14 @@ class RenderSliverList extends RenderSliverMultiBoxAdaptor {
       from: childScrollOffset(firstChild),
       to: endScrollOffset,
     );
+    final double targetEndScrollOffsetForPaint = constraints.scrollOffset + constraints.remainingPaintExtent;
     geometry = new SliverGeometry(
       scrollExtent: estimatedMaxScrollOffset,
       paintExtent: paintExtent,
       cacheExtent: cacheExtent,
       maxPaintExtent: estimatedMaxScrollOffset,
       // Conservative to avoid flickering away the clip during scroll.
-      hasVisualOverflow: endScrollOffset > targetEndScrollOffset || constraints.scrollOffset > 0.0,
+      hasVisualOverflow: endScrollOffset > targetEndScrollOffsetForPaint || constraints.scrollOffset > 0.0,
     );
 
     // We may have started the layout while scrolled to the end, which would not

packages/flutter/test/widgets/list_view_test.dart

@@ -5,6 +5,8 @@
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
+import '../rendering/mock_canvas.dart';
+
 class TestSliverChildListDelegate extends SliverChildListDelegate {
   TestSliverChildListDelegate(List<Widget> children) : super(children);
 
@@ -293,4 +295,114 @@ void main() {
     // Leave left/right padding as is for children.
     expect(innerMediaQueryPadding, const EdgeInsets.symmetric(horizontal: 30.0));
   });
+
+  testWidgets('ListView clips if overflow is smaller than cacheExtent', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/17426.
+
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new Center(
+          child: new Container(
+            height: 200.0,
+            child: new ListView(
+              cacheExtent: 500.0,
+              children: <Widget>[
+                new Container(
+                  height: 90.0,
+                ),
+                new Container(
+                  height: 110.0,
+                ),
+                new Container(
+                  height: 80.0,
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(find.byType(Viewport), paints..clipRect());
+  });
+
+  testWidgets('ListView does not clips if no overflow', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new Center(
+          child: new Container(
+            height: 200.0,
+            child: new ListView(
+              cacheExtent: 500.0,
+              children: <Widget>[
+                new Container(
+                  height: 100.0,
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+  );
+
+    expect(find.byType(Viewport), isNot(paints..clipRect()));
+  });
+
+  testWidgets('ListView (fixed extent) clips if overflow is smaller than cacheExtent', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/17426.
+
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new Center(
+          child: new Container(
+            height: 200.0,
+            child: new ListView(
+              itemExtent: 100.0,
+              cacheExtent: 500.0,
+              children: <Widget>[
+                new Container(
+                  height: 100.0,
+                ),
+                new Container(
+                  height: 100.0,
+                ),
+                new Container(
+                  height: 100.0,
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(find.byType(Viewport), paints..clipRect());
+  });
+
+  testWidgets('ListView (fixed extent) does not clips if no overflow', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new Center(
+          child: new Container(
+            height: 200.0,
+            child: new ListView(
+              itemExtent: 100.0,
+              cacheExtent: 500.0,
+              children: <Widget>[
+                new Container(
+                  height: 100.0,
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(find.byType(Viewport), isNot(paints..clipRect()));
+  });
 }