Adapt and reland old position sticky implementation

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

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index 6dc9581abac44e64276b3d543581cd8684337713..665c23f07c336dfd7a9896fff59cb587ceb11416 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -1556,7 +1556,7 @@ void FrameView::didScrollTimerFired(Timer<FrameView>*)
         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())
@@ -1564,6 +1564,14 @@ void FrameView::updateLayersAndCompositingAfterScrollIfNeeded()
 
     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.
@@ -2114,7 +2122,7 @@ void FrameView::scrollTo(const DoublePoint& newPosition)
         m_pendingScrollDelta += scrollDelta;
 
     clearScrollAnchor();
-    updateLayersAndCompositingAfterScrollIfNeeded();
+    updateLayersAndCompositingAfterScrollIfNeeded(scrollDelta);
     scrollPositionChanged();
     frame().loader().client()->didChangeScrollOffset();
 }