Correct bottom overscroll in VariableHeightScrollable

sky/sdk/lib/widgets/variable_height_scrollable.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:collection';
 
 import 'package:sky/animation/scroll_behavior.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/block_viewport.dart';
 import 'package:sky/widgets/scrollable.dart';
 import 'package:sky/widgets/widget.dart';
 
 class VariableHeightScrollable extends Scrollable {
   VariableHeightScrollable({
     String key,
     this.builder,
     this.token
   }) : super(key: key);
 
   IndexedBuilder builder;
   Object token;
+  bool _contentsChanged = true;
 
   void syncFields(VariableHeightScrollable source) {
     builder = source.builder;
+    if (token != source.token)
+      _contentsChanged = true;
     token = source.token;
     super.syncFields(source);
   }
 
   ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
   OverscrollBehavior get scrollBehavior => super.scrollBehavior;
 
   void _handleSizeChanged(Size newSize) {
     scrollBehavior.containerSize = newSize.height;
   }
 
   void _handleLayoutChanged(
     int firstVisibleChildIndex,
     int visibleChildCount,
     UnmodifiableListView<double> childOffsets,
     bool didReachLastChild
   ) {
     assert(childOffsets.length > 0);
-    scrollBehavior.contentsSize = didReachLastChild ? childOffsets.last : double.INFINITY;
-    if (didReachLastChild && scrollOffset > scrollBehavior.maxScrollOffset)
-      settleScrollOffset();
+    if (didReachLastChild) {
+      scrollBehavior.contentsSize = childOffsets.last;
+      if (_contentsChanged && scrollOffset > scrollBehavior.maxScrollOffset) {
+        _contentsChanged = false;
+        settleScrollOffset();
+      }
+    } else {
+      scrollBehavior.contentsSize = double.INFINITY;
+    }
   }
 
   Widget buildContent() {
     return new SizeObserver(
       callback: _handleSizeChanged,
       child: new BlockViewport(
         builder: builder,
         onLayoutChanged: _handleLayoutChanged,
         startOffset: scrollOffset,
         token: token
       )
     );
   }
 }