Adapt and reland old position sticky implementation

third_party/WebKit/Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 1538 matching lines @@
     layoutView()->clearHitTestCache();
     frame().loader().saveScrollState();
 }
 
 void FrameView::didScrollTimerFired(Timer<FrameView>*)
 {
     if (m_frame->document() && m_frame->document()->layoutView())
         m_frame->document()->fetcher()->updateAllImageResourcePriorities();
 }
 
-void FrameView::updateLayersAndCompositingAfterScrollIfNeeded()
+void FrameView::updateLayersAndCompositingAfterScrollIfNeeded(const DoubleSize& scrollDelta)
 {
     // Nothing to do after scrolling if there are no fixed position elements.
     if (!hasViewportConstrainedObjects())
         return;
 
     RefPtrWillBeRawPtr<FrameView> protect(this);
 
+    // Update sticky position objects which are stuck to the viewport.
+    for (const auto& viewportConstrainedObject : *m_viewportConstrainedObjects) {

[chrishtr, 2015/11/04 01:55:18]

Shouldn't this go in PaintLayerScrollableArea? This kind of code needs to run
for scrollers that are not the view also..

[flackr, 2015/11/25 20:47:52]

The root frame doesn't use PaintLayerScrollableArea as far as I can tell. The
ScrollableArea for the root frame is a RootFrameViewport.

Scrollers that are not the root frame view are actually updated by
PaintLayerScrollableArea::setScrollOffset when it calls
updateLayerPositionsAfterOverflowScroll(scrollDelta).

[chrishtr, 2015/12/09 00:37:38]

What about iframes?

+        LayoutObject* layoutObject = viewportConstrainedObject;
+        PaintLayer* layer = toLayoutBoxModelObject(layoutObject)->layer();
+        if (layoutObject->style()->position() == StickyPosition)
+            layer->updateLayerPositionAfterFrameScroll(scrollDelta);
+    }
+
     // If there fixed position elements, scrolling may cause compositing layers to change.
     // Update widget and layer positions after scrolling, but only if we're not inside of
     // layout.
     if (!m_nestedLayoutCount) {
         updateWidgetPositions();
         if (LayoutView* layoutView = this->layoutView())
             layoutView->layer()->setNeedsCompositingInputsUpdate();
     }
 }
 
@@ skipping 530 matching lines @@
         // Don't scroll the FrameView!
         ASSERT_NOT_REACHED();
     }
 
     m_scrollPosition = newPosition;
 
     if (!scrollbarsSuppressed())
         m_pendingScrollDelta += scrollDelta;
 
     clearScrollAnchor();
-    updateLayersAndCompositingAfterScrollIfNeeded();
+    updateLayersAndCompositingAfterScrollIfNeeded(scrollDelta);
     scrollPositionChanged();
     frame().loader().client()->didChangeScrollOffset();
 }
 
 void FrameView::invalidatePaintForTickmarks() const
 {
     if (Scrollbar* scrollbar = verticalScrollbar())
         scrollbar->invalidate();
 }
 
@@ skipping 1950 matching lines @@
 }
 
 bool FrameView::canThrottleRendering() const
 {
     if (!RuntimeEnabledFeatures::renderingPipelineThrottlingEnabled())
         return false;
     return m_hiddenForThrottling && m_crossOriginForThrottling;
 }
 
 } // namespace blink