Handle nested position:sticky elements correctly (compositor)

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 154 matching lines @@
          layoutObject.isVideo();
 }
 
 // Get the scrolling coordinator in a way that works inside
 // CompositedLayerMapping's destructor.
 static ScrollingCoordinator* scrollingCoordinatorFromLayer(PaintLayer& layer) {
   Page* page = layer.layoutObject().frame()->page();
   return (!page) ? nullptr : page->scrollingCoordinator();
 }
 
+static WebLayer* nearestCompositedStickyLayer(
+    const StickyConstraintsMap& constraintsMap,
+    LayoutBoxModelObject* obj) {
+  while (obj && obj->compositingState() == NotComposited) {

[flackr, 2017/03/09 20:22:09]

I think technically you want to find compositingState() == PaintsIntoOwnBacking,
but it's probably better / more consistent to just check if the object has a
compositedLayerMapping since that's what we intend to access.

[smcgruer, 2017/03/10 16:37:10]

Done.

+    DCHECK(constraintsMap.contains(obj->layer()));
+    const StickyPositionScrollingConstraints constraints =
+        constraintsMap.at(obj->layer());
+    obj = constraints.nearestStickyBoxShiftingStickyBox()

[flackr, 2017/03/09 20:22:08]

This seems wrong, don't we need to follow the same chain as the one we started
on? i.e. if we're interested in nearestStickyBoxShiftingContainingBlock then
only follow those pointers?

[smcgruer, 2017/03/14 22:04:06]

I believe you're correct, done. Adding a test is waiting on resolving copying
back pointers to main from cc.

+              ? constraints.nearestStickyBoxShiftingStickyBox()
+              : constraints.nearestStickyBoxShiftingContainingBlock();
+  }
+
+  if (!obj)
+    return nullptr;
+
+  return obj->layer()
+      ->compositedLayerMapping()
+      ->mainGraphicsLayer()
+      ->platformLayer();
+}
+
 CompositedLayerMapping::CompositedLayerMapping(PaintLayer& layer)
     : m_owningLayer(layer),
       m_contentOffsetInCompositingLayerDirty(false),
       m_pendingUpdateScope(GraphicsLayerUpdateNone),
       m_isMainFrameLayoutViewLayer(false),
       m_backgroundLayerPaintsFixedRootBackground(false),
       m_scrollingContentsAreEmpty(false),
       m_backgroundPaintsOntoScrollingContentsLayer(false),
       m_backgroundPaintsOntoGraphicsLayer(false),
       m_drawsBackgroundOntoContentLayer(false) {
@@ skipping 115 matching lines @@
   if (sticky) {
     if (!ancestorOverflowLayer->isRootLayer()) {
       sticky = ancestorOverflowLayer->needsCompositedScrolling();
     } else {
       sticky = layoutObject().view()->frameView()->isScrollable();
     }
   }
 
   WebLayerStickyPositionConstraint webConstraint;
   if (sticky) {
+    const StickyConstraintsMap& constraintsMap =
+        ancestorOverflowLayer->getScrollableArea()->stickyConstraintsMap();
     const StickyPositionScrollingConstraints& constraints =
-        ancestorOverflowLayer->getScrollableArea()->stickyConstraintsMap().at(
-            &m_owningLayer);
+        constraintsMap.at(&m_owningLayer);
 
     // Find the layout offset of the unshifted sticky box within its
     // compositingContainer. If the enclosing layer is not the scroller, then
     // the offset must be adjusted to include the scroll offset to keep it
     // relative to compositingContainer.
     LayoutPoint enclosingLayerOffset;
     compositingContainer->convertToLayerCoords(ancestorOverflowLayer,
                                                enclosingLayerOffset);
     if (compositingContainer != ancestorOverflowLayer) {
       enclosingLayerOffset += LayoutSize(
@@ skipping 22 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());
-    // TODO(smcgruer): Copy fields for nested sticky in cc (crbug.com/672710)
+    webConstraint.nearestLayerShiftingStickyBox = nearestCompositedStickyLayer(
+        constraintsMap, constraints.nearestStickyBoxShiftingStickyBox());
+    webConstraint.nearestLayerShiftingContainingBlock =
+        nearestCompositedStickyLayer(
+            constraintsMap,
+            constraints.nearestStickyBoxShiftingContainingBlock());
+    webConstraint.localStickyOffset = constraints.getLocalStickyOffset();

[flackr, 2017/03/09 20:22:09]

This seems to be passed in, but unused as far as I can tell. I'm inclined to
think that we shouldn't pass this in since it changes with every scroll but we
don't update the constraints every scroll.

[smcgruer, 2017/03/10 16:37:10]

Its used in draw_property_utils::CalculateTotalStickyOffsetToScroller. We need
it to adjust the main_thread_offset.

[flackr, 2017/03/13 19:49:45]

As discussed, we should try to work out the local offset on the compositor side
since it will change with calls to Layer::SetPosition without the sticky
constraints being updated.

[smcgruer, 2017/03/14 22:04:06]

Done.

   }
 
   m_graphicsLayer->setStickyPositionConstraint(webConstraint);
 }
 
 void CompositedLayerMapping::updateLayerBlendMode(const ComputedStyle& style) {
   setBlendMode(style.blendMode());
 }
 
 void CompositedLayerMapping::updateIsRootForIsolatedGroup() {
@@ skipping 3121 matching lines @@
   } else if (graphicsLayer == m_decorationOutlineLayer.get()) {
     name = "Decoration Layer";
   } else {
     ASSERT_NOT_REACHED();
   }
 
   return name;
 }
 
 }  // namespace blink