Fix virtual-collection header flickering

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 135 matching lines @@
       if (index >= 0) {
         final minScrollTop = _itemHeight * (index + 1) - _height;
         final maxScrollTop = _itemHeight * index;
         scrollTop = ((maxScrollTop - minScrollTop) / 2 + minScrollTop).floor();
       }
       _takeIntoView = null;
     }
 
     final top = (scrollTop / _itemHeight).floor();
 
-    _header.style.top = '${scrollTop}px';
+    _updateHeader();
     _scroller.style.height = '${_itemHeight*(_items.length)+_headerHeight}px';
     final tail_length = (_height / _itemHeight / _preload).ceil();
     final length = tail_length * 2 + tail_length * _preload;
 
     if (_container.children.length < length) {
       while (_container.children.length != length) {
         var e = _create();
         e..style.display = 'hidden';
         _container.children.add(e);
       }
       _top = null; // force update;
     }
 
     if ((_top == null) || ((top - _top).abs() >= tail_length)) {
       _shifter.style.top = '${_itemHeight*(top-tail_length)}px';
       int i = top - tail_length;
       for (final HtmlElement e in _container.children) {
         if (0 <= i && i < _items.length) {
           e..style.display = null;
           _update(e, _items[i], i);
         } else {
           e.style.display = 'hidden';
         }
         i++;
       }
       _top = top;
     }
   }
 
+  void _updateHeader() {
+    _header.style.top = '${scrollTop}px';
+  }
+
   void _onScroll(_) {
+    // needed to avoid flickering
+    _updateHeader();
     _r.dirty();
   }
 
   void _onResize(_) {
     final newHeight = getBoundingClientRect().height;
     if (newHeight > _height) {
       _height = newHeight;
       _r.dirty();
     }
   }
 }