Make RenderBlock painting match its hittesting

sky/sdk/lib/rendering/block.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:math' as math;
 
 import 'box.dart';
 import 'object.dart';
 
 class BlockParentData extends BoxParentData with ContainerParentDataMixin<RenderBox> { }
 
 class RenderBlock extends RenderBox with ContainerRenderObjectMixin<RenderBox, BlockParentData>,
                                          RenderBoxContainerDefaultsMixin<RenderBox, BlockParentData> {
   // lays out RenderBox children in a vertical stack
   // uses the maximum width provided by the parent
   // sizes itself to the height of its child stack
 
+  bool _hasVisualOverflow = false;
+
   RenderBlock({
     List<RenderBox> children
   }) {
     addAll(children);
   }
 
   void setupParentData(RenderBox child) {
     if (child.parentData is! BlockParentData)
       child.parentData = new BlockParentData();
   }
@@ skipping 59 matching lines @@
     BoxConstraints innerConstraints = _getInnerConstraintsForWidth(width);
     double y = 0.0;
     RenderBox child = firstChild;
     while (child != null) {
       child.layout(innerConstraints, parentUsesSize: true);
       assert(child.parentData is BlockParentData);
       child.parentData.position = new Point(0.0, y);
       y += child.size.height;
       child = child.parentData.nextSibling;
     }
+    // FIXME(eseidel): Block lays out its children with unconstrained height
+    // yet itself remains constrained. Remember that our children wanted to
+    // be taller than we are so we know to clip them (and not cause confusing
+    // mismatch of painting vs. hittesting).
     size = new Size(width, constraints.constrainHeight(y));
+    _hasVisualOverflow = y > size.height;
     assert(!size.isInfinite);
   }
 
   void hitTestChildren(HitTestResult result, { Point position }) {
     defaultHitTestChildren(result, position: position);
   }
 
   void paint(PaintingCanvas canvas, Offset offset) {
+    if (_hasVisualOverflow) {
+      canvas.save();
+      canvas.clipRect(offset & size);
+    }
     defaultPaint(canvas, offset);
+    if (_hasVisualOverflow) {
+      canvas.restore();
+    }
   }
 
 }