Reland main thread position sticky implementation.

third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp

 /*
  * Copyright (C) 2011 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 919 matching lines @@
 #endif
 
 bool ScrollingCoordinator::hasVisibleSlowRepaintViewportConstrainedObjects(FrameView* frameView) const
 {
     const FrameView::ViewportConstrainedObjectSet* viewportConstrainedObjects = frameView->viewportConstrainedObjects();
     if (!viewportConstrainedObjects)
         return false;
 
     for (const LayoutObject* layoutObject : *viewportConstrainedObjects) {
         ASSERT(layoutObject->isBoxModelObject() && layoutObject->hasLayer());
-        ASSERT(layoutObject->style()->position() == FixedPosition);
+        ASSERT(layoutObject->style()->position() == FixedPosition
+            || layoutObject->style()->position() == StickyPosition);
         PaintLayer* layer = toLayoutBoxModelObject(layoutObject)->layer();
 
         // Whether the Layer scrolls with the viewport is a tree-depenent
         // property and our viewportConstrainedObjects collection is maintained
         // with only LayoutObject-level information.
         if (!layer->scrollsWithViewport())
             continue;
 
         // If the whole subtree is invisible, there's no reason to scroll on
         // the main thread because we don't need to generate invalidations
@@ skipping 28 matching lines @@
     for (Frame* frame = m_page->mainFrame(); frame; frame = frame->tree().traverseNext()) {
         if (!frame->isLocalFrame())
             continue;
 
         FrameView* frameView = toLocalFrame(frame)->view();
         if (!frameView || frameView->shouldThrottleRendering())
             continue;
 
         if (frameView->hasBackgroundAttachmentFixedObjects())
             reasons |= MainThreadScrollingReason::kHasBackgroundAttachmentFixedObjects;
+        if (frameView->hasStickyPositionObjects())
+            reasons |= MainThreadScrollingReason::kHasStickyPositionObjects;
         FrameView::ScrollingReasons scrollingReasons = frameView->getScrollingReasons();
         const bool mayBeScrolledByInput = (scrollingReasons == FrameView::Scrollable);
         const bool mayBeScrolledByScript = mayBeScrolledByInput || (scrollingReasons ==
             FrameView::NotScrollableExplicitlyDisabled);
 
         // TODO(awoloszyn) Currently crbug.com/304810 will let certain
         // overflow:hidden elements scroll on the compositor thread, so we should
         // not let this move there path as an optimization, when we have slow-repaint
         // elements.
         if (mayBeScrolledByScript && hasVisibleSlowRepaintViewportConstrainedObjects(frameView)) {
             reasons |= MainThreadScrollingReason::kHasNonLayerViewportConstrainedObjects;
         }
     }
 
     return reasons;
 }
 
 String ScrollingCoordinator::mainThreadScrollingReasonsAsText(MainThreadScrollingReasons reasons)
 {
     StringBuilder stringBuilder;
 
     if (reasons & MainThreadScrollingReason::kHasBackgroundAttachmentFixedObjects)
         stringBuilder.appendLiteral("Has background-attachment:fixed, ");
     if (reasons & MainThreadScrollingReason::kHasNonLayerViewportConstrainedObjects)
         stringBuilder.appendLiteral("Has non-layer viewport-constrained objects, ");
+    if (reasons & MainThreadScrollingReason::kHasStickyPositionObjects)
+        stringBuilder.appendLiteral("Has sticky position objects, ");
     if (reasons & MainThreadScrollingReason::kThreadedScrollingDisabled)
         stringBuilder.appendLiteral("Threaded scrolling is disabled, ");
     if (reasons & MainThreadScrollingReason::kAnimatingScrollOnMainThread)
         stringBuilder.appendLiteral("Animating scroll on main thread, ");
 
     if (stringBuilder.length())
         stringBuilder.resize(stringBuilder.length() - 2);
     return stringBuilder.toString();
 }
 
@@ skipping 12 matching lines @@
     bool frameIsScrollable = frameView && frameView->isScrollable();
     if (frameIsScrollable != m_wasFrameScrollable)
         return true;
 
     if (WebLayer* scrollLayer = frameView ? toWebLayer(frameView->layerForScrolling()) : nullptr)
         return WebSize(frameView->contentsSize()) != scrollLayer->bounds();
     return false;
 }
 
 } // namespace blink