Revert of DevTools: fix stick to bottom in console viewport

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 b8edc115a9ee4291e06b2e6009042ec172aa4131..4774a14ae7dce4b0afb8b21e4199ece8f6738f12 100644
--- a/third_party/WebKit/Source/devtools/front_end/ui/ViewportControl.js
+++ b/third_party/WebKit/Source/devtools/front_end/ui/ViewportControl.js
@@ -59,13 +59,9 @@
     this._renderedItems = [];
     this._anchorSelection = null;
     this._headSelection = null;
+    this._stickToBottom = false;
+    this._scrolledToBottom = true;
     this._itemCount = 0;
-
-    // Listen for any changes to descendants and trigger a refresh. This ensures
-    // that items updated asynchronously will not break stick-to-bottom behavior
-    // if they change the scroll height.
-    this._observer = new MutationObserver(this.refresh.bind(this));
-    this._observerConfig = { childList: true, subtree: true };
 }
 
 /**
@@ -149,9 +145,9 @@
     /**
      * @return {boolean}
      */
-    stickToBottom: function()
-    {
-        return this._stickToBottom;
+    scrolledToBottom: function()
+    {
+        return this._scrolledToBottom;
     },
 
     /**
@@ -160,10 +156,6 @@
     setStickToBottom: function(value)
     {
         this._stickToBottom = value;
-        if (this._stickToBottom)
-            this._observer.observe(this._contentElement, this._observerConfig);
-        else
-            this._observer.disconnect();
     },
 
     /**
@@ -384,14 +376,6 @@
 
     refresh: function()
     {
-        this._observer.disconnect();
-        this._innerRefresh();
-        if (this._stickToBottom)
-            this._observer.observe(this._contentElement, this._observerConfig);
-    },
-
-    _innerRefresh: function()
-    {
         if (!this._visibleHeight())
             return;  // Do nothing for invisible controls.
 
@@ -412,6 +396,7 @@
 
         var visibleFrom = this.element.scrollTop;
         var visibleHeight = this._visibleHeight();
+        this._scrolledToBottom = this.element.isScrolledToBottom();
         var isInvalidating = !this._cumulativeHeights;
 
         for (var i = 0; i < this._renderedItems.length; ++i) {
@@ -423,19 +408,17 @@
         var oldFirstVisibleIndex = this._firstVisibleIndex;
         var oldLastVisibleIndex = this._lastVisibleIndex;
 
-        // When the viewport is scrolled to the bottom, using the cumulative heights estimate is not
-        // precise enough to determine next visible indices. This stickToBottom check avoids extra
-        // calls to refresh in those cases.
-        if (this._stickToBottom) {
+        var shouldStickToBottom = this._stickToBottom && this._scrolledToBottom;
+
+        if (shouldStickToBottom) {
+            this._lastVisibleIndex = this._itemCount - 1;
             this._firstVisibleIndex = Math.max(this._itemCount - Math.ceil(visibleHeight / this._provider.minimumRowHeight()), 0);
-            this._lastVisibleIndex = this._itemCount - 1;
         } 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, this._itemCount - 1);
         }
-
         var topGapHeight = this._cumulativeHeights[this._firstVisibleIndex - 1] || 0;
         var bottomGapHeight = this._cumulativeHeights[this._cumulativeHeights.length - 1] - this._cumulativeHeights[this._lastVisibleIndex];
 
@@ -459,7 +442,7 @@
         // Should be the last call in the method as it might force layout.
         if (shouldRestoreSelection)
             this._restoreSelection(selection);
-        if (this._stickToBottom)
+        if (shouldStickToBottom)
             this.element.scrollTop = 10000000;
     },
 
@@ -629,11 +612,8 @@
      */
     forceScrollItemToBeFirst: function(index)
     {
-        this.setStickToBottom(false);
         this._rebuildCumulativeHeightsIfNeeded();
         this.element.scrollTop = index > 0 ? this._cumulativeHeights[index - 1] : 0;
-        if (this.element.isScrolledToBottom())
-            this.setStickToBottom(true);
         this.refresh();
     },
 
@@ -642,11 +622,8 @@
      */
     forceScrollItemToBeLast: function(index)
     {
-        this.setStickToBottom(false);
         this._rebuildCumulativeHeightsIfNeeded();
         this.element.scrollTop = this._cumulativeHeights[index] - this._visibleHeight();
-        if (this.element.isScrolledToBottom())
-            this.setStickToBottom(true);
         this.refresh();
     },