Handle nested position:sticky elements

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

 /*
  * Copyright (C) 2009, 2010, 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 268 matching lines @@
 void CompositedLayerMapping::updateFilters(const ComputedStyle& style) {
   m_graphicsLayer->setFilters(
       owningLayer().createCompositorFilterOperationsForFilter(style));
 }
 
 void CompositedLayerMapping::updateBackdropFilters(const ComputedStyle& style) {
   m_graphicsLayer->setBackdropFilters(
       owningLayer().createCompositorFilterOperationsForBackdropFilter(style));
 }
 
+namespace {
+WebLayer* webLayerFromLayoutBoxModelObject(LayoutBoxModelObject* obj) {
+  if (!obj) {

[flackr, 2017/02/02 18:18:11]

nit: no curlies { } on single line if blocks.

[smcgruer, 2017/02/02 20:21:27]

Done.

+    return nullptr;
+  }
+
+  PaintLayer* paintLayer = obj->layer();
+  if (!paintLayer) {
+    return nullptr;
+  }
+
+  CompositedLayerMapping* mapping = paintLayer->compositedLayerMapping();
+  if (!mapping) {
+    return nullptr;
+  }
+
+  GraphicsLayer* graphicsLayer = mapping->mainGraphicsLayer();
+  if (!graphicsLayer) {
+    return nullptr;
+  }
+
+  return graphicsLayer->platformLayer();
+}
+}  // namespace
+
 void CompositedLayerMapping::updateStickyConstraints(
     const ComputedStyle& style) {
   bool sticky = style.position() == EPosition::StickyPosition;
   const PaintLayer* ancestorOverflowLayer =
       m_owningLayer.ancestorOverflowLayer();
   // TODO(flackr): Do we still need this?
   if (sticky) {
     if (!ancestorOverflowLayer->isRootLayer()) {
       sticky = ancestorOverflowLayer->needsCompositedScrolling();
     } else {
@@ skipping 35 matching lines @@
     webConstraint.leftOffset = constraints.leftOffset();
     webConstraint.rightOffset = constraints.rightOffset();
     webConstraint.topOffset = constraints.topOffset();
     webConstraint.bottomOffset = constraints.bottomOffset();
     webConstraint.parentRelativeStickyBoxOffset =
         roundedIntPoint(stickyBoxOffset);
     webConstraint.scrollContainerRelativeStickyBoxRect =
         enclosingIntRect(constraints.scrollContainerRelativeStickyBoxRect());
     webConstraint.scrollContainerRelativeContainingBlockRect = enclosingIntRect(
         constraints.scrollContainerRelativeContainingBlockRect());
+    webConstraint.nearestStickyElementShiftingStickyBox =
+        webLayerFromLayoutBoxModelObject(
+            constraints.nearestStickyElementShiftingStickyBox());

[flackr, 2017/02/02 18:18:11]

If the nearest sticky element is not composited we have to point to the next one
(and so on) to correctly include their composited sticky offset.

[smcgruer, 2017/02/02 20:21:27]

I can have a go at implementing this behavior, but we may have to whiteboard
this one before I understand it :). Not clear how this could happen or why we
would want to end up accumulating offset from something further up the tree...

[flackr, 2017/02/02 23:31:15]

On low dpi this is as simple as
<div id='outer' style='position: sticky; will-change: transform;'>
  <div id='middle' style='position: sticky;'>
    <div id='inner' style='position: sticky; will-change: transform;'>
    </div>
  </div>
</div>

#inner's constraint rects will be affected by the main thread update of #middle,
but also by the compositing position update of #outer.

[smcgruer, 2017/02/07 19:05:26]

Thanks for the repro case. Done except I think I need to use something other
than containing block.

+    webConstraint.nearestStickyElementShiftingContainingBlock =
+        webLayerFromLayoutBoxModelObject(
+            constraints.nearestStickyElementShiftingContainingBlock());
   }
 
   m_graphicsLayer->setStickyPositionConstraint(webConstraint);
 }
 
 void CompositedLayerMapping::updateLayerBlendMode(const ComputedStyle& style) {
   setBlendMode(style.blendMode());
 }
 
 void CompositedLayerMapping::updateIsRootForIsolatedGroup() {
@@ skipping 3116 matching lines @@
   } else if (graphicsLayer == m_decorationOutlineLayer.get()) {
     name = "Decoration Layer";
   } else {
     ASSERT_NOT_REACHED();
   }
 
   return name;
 }
 
 }  // namespace blink