DevTools: [Console] fix viewport stick-to-bottom behavior

third_party/WebKit/Source/devtools/front_end/ui/ViewportControl.js

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 360 matching lines @@
         }
 
         selection.setBaseAndExtent(anchorElement, anchorOffset, headElement, headOffset);
     },
 
     refresh: function()
     {
         if (!this._visibleHeight())
             return;  // Do nothing for invisible controls.
 
-        var itemCount = this._provider.itemCount();
-        if (!itemCount) {
+        if (!this._provider.itemCount()) {
             for (var i = 0; i < this._renderedItems.length; ++i)
                 this._renderedItems[i].willHide();
             this._renderedItems = [];
             this._contentElement.removeChildren();
             this._topGapElement.style.height = "0px";
             this._bottomGapElement.style.height = "0px";
             this._firstVisibleIndex = -1;
             this._lastVisibleIndex = -1;
             return;
         }
 
         var selection = this.element.getComponentSelection();
         var shouldRestoreSelection = this._updateSelectionModel(selection);
 
         var visibleFrom = this.element.scrollTop;
         var visibleHeight = this._visibleHeight();
         this._scrolledToBottom = this.element.isScrolledToBottom();
         var isInvalidating = !this._cumulativeHeights;
 
-        if (this._cumulativeHeights && itemCount !== this._cumulativeHeights.length)
-            delete this._cumulativeHeights;
         for (var i = 0; i < this._renderedItems.length; ++i) {
             // Tolerate 1-pixel error due to double-to-integer rounding errors.
             if (this._cumulativeHeights && Math.abs(this._cachedItemHeight(this._firstVisibleIndex + i) - this._renderedItems[i].element().offsetHeight) > 1)
                 delete this._cumulativeHeights;
         }
         this._rebuildCumulativeHeightsIfNeeded();
+        var itemCount = this._cumulativeHeights.length;
         var oldFirstVisibleIndex = this._firstVisibleIndex;
         var oldLastVisibleIndex = this._lastVisibleIndex;
 
-        var shouldStickToBottom = this._stickToBottom && this._scrolledToBottom;
+        var shouldStickToBottom = isInvalidating && this._stickToBottom && this._scrolledToBottom;
+
         if (shouldStickToBottom) {
             this._lastVisibleIndex = itemCount - 1;
             this._firstVisibleIndex = Math.max(itemCount - Math.ceil(visibleHeight / this._provider.minimumRowHeight()), 0);
         } else {
             this._firstVisibleIndex = Math.max(Array.prototype.lowerBound.call(this._cumulativeHeights, visibleFrom + 1), 0);
             // Proactively render more rows in case some of them will be collapsed without triggering refresh. @see crbug.com/390169
             this._lastVisibleIndex = this._firstVisibleIndex + Math.ceil(visibleHeight / this._provider.minimumRowHeight()) - 1;
             this._lastVisibleIndex = Math.min(this._lastVisibleIndex, itemCount - 1);
         }
         var topGapHeight = this._cumulativeHeights[this._firstVisibleIndex - 1] || 0;
@@ skipping 206 matching lines @@
 
     /**
      * @return {number}
      */
     _visibleHeight: function()
     {
         // Use offsetHeight instead of clientHeight to avoid being affected by horizontal scroll.
         return this.element.offsetHeight;
     }
 }