Adapt and reland old position sticky implementation

Source/core/frame/FrameView.cpp

Index: Source/core/frame/FrameView.cpp
diff --git a/Source/core/frame/FrameView.cpp b/Source/core/frame/FrameView.cpp
index efcdd6c3a22a18d209f366da8f030030480bf837..3582b3597ada38dd3ec884e9bbe7681b868e4123 100644
--- a/Source/core/frame/FrameView.cpp
+++ b/Source/core/frame/FrameView.cpp
@@ -1563,7 +1563,7 @@ void FrameView::didScrollTimerFired(Timer<FrameView>*)
     }
 }
 
-void FrameView::updateLayersAndCompositingAfterScrollIfNeeded()
+void FrameView::updateLayersAndCompositingAfterScrollIfNeeded(const DoubleSize& scrollDelta)
 {
     // Nothing to do after scrolling if there are no fixed position elements.
     if (!hasViewportConstrainedObjects())
@@ -1571,6 +1571,14 @@ void FrameView::updateLayersAndCompositingAfterScrollIfNeeded()
 
     RefPtrWillBeRawPtr<FrameView> protect(this);
 
+    // Update sticky positioned elements which scroll with the viewport.
+    for (const auto& viewportConstrainedObject : *m_viewportConstrainedObjects) {
+        LayoutObject* layoutObject = viewportConstrainedObject;
+        DeprecatedPaintLayer* layer = toLayoutBoxModelObject(layoutObject)->layer();
+        if (layoutObject->style()->position() == StickyPosition && layer->scrollsWithViewport())
+            layer->updateLayerPositionsAfterOverflowScroll(scrollDelta);

[flackr, 2015/09/15 21:13:11]

+skobes, does this seem like the right way / place to update the viewport
constrained sticky elements after a frame view scroll?

[skobes, 2015/09/16 00:07:57]

Possibly, but keep in mind that FrameView scrolls are going away.  With root
layer scrolling you will have to hook
DeprecatedPaintLayerScrollableArea::setScrollOffset.

Related: can sticky-positioned elements be sticky inside overflow scrollers?  Or
do they only stick to the viewport?

[chrishtr, 2015/09/17 01:18:56]

This method of implementation is not performant enough. Iterating over all of
the position objects will take a long time, and has been a performance
bottleneck in the past.
The nearest scrolling ancestor:
https://drafts.csswg.org/css-position/Overview.html#sticky-pos

[flackr, 2015/09/22 22:25:32]

Agreed, my intention is to only add those sticky objects whose nearest ancestor
scroller is the FrameView to the FrameView's viewport constrained objects.
Currently, this information is not available when the style changes but I
believe Ojan was okay with propagating nearest ancestor scroller down during the
style recalc. Do you think this needs to be resolved before landing anything?

+    }
+
     // 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.
@@ -2129,7 +2137,7 @@ void FrameView::scrollTo(const DoublePoint& newPosition)
         m_pendingScrollDelta += scrollDelta;
 
     clearScrollAnchor();
-    updateLayersAndCompositingAfterScrollIfNeeded();
+    updateLayersAndCompositingAfterScrollIfNeeded(scrollDelta);
     scrollPositionChanged();
     frame().loader().client()->didChangeScrollOffset();
 }
@@ -3157,14 +3165,28 @@ int FrameView::scrollSize(ScrollbarOrientation orientation) const
     return scrollbar->totalSize() - scrollbar->visibleSize();
 }
 
-void FrameView::setScrollOffset(const IntPoint& offset, ScrollType)
+void FrameView::setScrollOffset(const IntPoint& offset, ScrollType type)
 {
-    scrollTo(clampScrollPosition(offset));
+    if (type != CompositorScroll) {
+        // Scrolling is triggered directly in response to scrollbar thumb movement,
+        // but should be processed after compositing update. http://crbug.com/420741
+        DisableCompositingQueryAsserts disabler;
+        scrollTo(clampScrollPosition(offset));
+    } else {
+        scrollTo(clampScrollPosition(offset));
+    }
 }
 
-void FrameView::setScrollOffset(const DoublePoint& offset, ScrollType)
+void FrameView::setScrollOffset(const DoublePoint& offset, ScrollType type)
 {
-    scrollTo(clampScrollPosition(offset));
+    if (type != CompositorScroll) {
+        // Scrolling is triggered directly in response to scrollbar thumb movement,
+        // but should be processed after compositing update. http://crbug.com/420741
+        DisableCompositingQueryAsserts disabler;
+        scrollTo(clampScrollPosition(offset));
+    } else {
+        scrollTo(clampScrollPosition(offset));
+    }
 }
 
 void FrameView::windowResizerRectChanged()