Implement position: sticky updates on compositor

third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/layout/compositing/CompositingReasonFinder.h"
 
 #include "core/CSSPropertyNames.h"
 #include "core/dom/Document.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/Settings.h"
@@ skipping 130 matching lines @@
         directReasons |= CompositingReasonOverflowScrollingTouch;
 
     // Composite |layer| if it is inside of an ancestor scrolling layer, but that
     // scrolling layer is not on the stacking context ancestor chain of |layer|.
     // See the definition of the scrollParent property in Layer for more detail.
     if (const PaintLayer* scrollingAncestor = layer->ancestorScrollingLayer()) {
         if (scrollingAncestor->needsCompositedScrolling() && layer->scrollParent())
             directReasons |= CompositingReasonOverflowScrollingParent;
     }
 
-    if (requiresCompositingForPositionFixed(layer))
-        directReasons |= CompositingReasonPositionFixed;
+    // TODO(flackr): Rename functions and variables to include sticky position (i.e. ScrollDependentPosition rather than PositionFixed).
+    if (requiresCompositingForScrollDependentPosition(layer))
+        directReasons |= CompositingReasonScrollDependentPosition;
 
     directReasons |= layoutObject->additionalCompositingReasons();
 
     ASSERT(!(directReasons & CompositingReasonComboAllStyleDeterminedReasons));
     return directReasons;
 }
 
 bool CompositingReasonFinder::requiresCompositingForAnimation(const ComputedStyle& style) const
 {
     if (style.subtreeWillChangeContents())
         return style.isRunningAnimationOnCompositor();
 
     return style.shouldCompositeForCurrentAnimations();
 }
 
-bool CompositingReasonFinder::requiresCompositingForPositionFixed(const PaintLayer* layer) const
+bool CompositingReasonFinder::requiresCompositingForScrollDependentPosition(const PaintLayer* layer) const
 {
     if (!(m_compositingTriggers & ViewportConstrainedPositionedTrigger))
         return false;
     // Don't promote fixed position elements that are descendants of a non-view container, e.g. transformed elements.
     // They will stay fixed wrt the container rather than the enclosing frame.
-    return layer->scrollsWithViewport() && m_layoutView.frameView()->isScrollable();
+    if (layer->scrollsWithViewport())
+        return m_layoutView.frameView()->isScrollable();
+    return layer->layoutObject()->style()->position() == StickyPosition && layer->ancestorOverflowLayer()->scrollsOverflow();
 }
 
 } // namespace blink