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

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

Index: third_party/WebKit/Source/devtools/front_end/ui/ViewportControl.js
diff --git a/third_party/WebKit/Source/devtools/front_end/ui/ViewportControl.js b/third_party/WebKit/Source/devtools/front_end/ui/ViewportControl.js
index ddb8a45fa8832a4ac66bf23f832ebf1435d421b1..fb920efd36231c0f209a8d3e89b031b26ea8aba1 100644
--- a/third_party/WebKit/Source/devtools/front_end/ui/ViewportControl.js
+++ b/third_party/WebKit/Source/devtools/front_end/ui/ViewportControl.js
@@ -378,8 +378,7 @@ WebInspector.ViewportControl.prototype = {
         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 = [];
@@ -399,18 +398,18 @@ WebInspector.ViewportControl.prototype = {
         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);