Fixed Observatory class-view crash

runtime/observatory/lib/src/elements/containers/virtual_collection.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. 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:async';
 import 'dart:html';
 import 'package:observatory/src/elements/helpers/rendering_scheduler.dart';
 import 'package:observatory/src/elements/helpers/tag.dart';
 
 typedef HtmlElement VirtualCollectionCreateCallback();
@@ skipping 88 matching lines @@
   /// L = l + 2 * l / _preload
   /// l = L * _preload / (_preload + 2)
   ///
   /// tail = l / _preload = L * 1 / (_preload + 2) = L * _inverse_preload
   static const double _inverse_preload = 1 / (_preload + 2);
 
   void render() {
     _top = (scrollTop / _itemHeight).floor();
 
     _scroller.style.height = '${_itemHeight*(_items.length)}px';
-    _shifter.style.top = '${_itemHeight*_top}px';
     final tail_length = (_height / _itemHeight / _preload).ceil();
+    _shifter.style.top = '${_itemHeight*(_top - tail_length)}px';
     final length = tail_length * 2 + tail_length * _preload;
 
     if (_shifter.children.length < length) {
       while (_shifter.children.length != length) {
         var e = _create();
         e..style.display = 'hidden';
         _shifter.children.add(e);
       }
-      _shifter.style.height = '${_itemHeight*length}px';
       children = [
         _scroller
           ..children = [_shifter]
       ];
     }
 
     int i = _top - tail_length;
     for (final HtmlElement e in _shifter.children) {
       if (0 <= i && i < _items.length) {
         e..style.display = null;
@@ skipping 15 matching lines @@
 
   void _onScroll(_) {
     if(_r.isDirty) return;
     if ((scrollTop - _top * _itemHeight).abs() >=
          _shifter.children.length * _inverse_preload * _itemHeight) {
       _r.dirty();
     }
   }
 
   void _onResize(_) {
-    _height = getBoundingClientRect().height;
-    _r.dirty();
+    final newHeight = getBoundingClientRect().height;
+    if (newHeight > _height) {
+      _height = newHeight;
+      _r.dirty();
+    }
   }
 }