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 290 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.
+    // Find the layout offset of the unshifted sticky box within its parent
+    // composited layer. This information is used by the compositor side to
+    // compute the additional offset required to keep the element stuck under
+    // compositor scrolling.
+    //
+    // Starting from the scroll container relative location, removing the
+    // enclosing layer's offset and the content offset in the composited layer
+    // results in the parent-layer relative offset.
+    FloatPoint parentRelativeStickyBoxOffset =
+        constraints.scrollContainerRelativeStickyBoxRect().location();
+
+    // The enclosing layers offset returned from |convertToLayerCoords| must be
+    // adjusted for both scroll and ancestor sticky elements.
     LayoutPoint enclosingLayerOffset;
     compositingContainer->convertToLayerCoords(ancestorOverflowLayer,
                                                enclosingLayerOffset);
     if (compositingContainer != ancestorOverflowLayer) {
       enclosingLayerOffset += LayoutSize(
           ancestorOverflowLayer->getScrollableArea()->getScrollOffset());
     }
+    // TODO(smcgruer): Until http://crbug.com/702229 is fixed, the nearest
+    // sticky ancestor may be non-composited which will make this offset wrong.
+    if (const LayoutBoxModelObject* ancestor =
+            constraints.nearestStickyAncestor()) {
+      enclosingLayerOffset -=
+          roundedIntSize(constraintsMap.at(ancestor->layer())
+                             .getTotalContainingBlockStickyOffset());
+    }
 
-    FloatPoint stickyBoxOffset =
-        constraints.scrollContainerRelativeStickyBoxRect().location();
     DCHECK(!m_contentOffsetInCompositingLayerDirty);
-    stickyBoxOffset.moveBy(FloatPoint(-enclosingLayerOffset) -
-                           FloatSize(contentOffsetInCompositingLayer()));
+    parentRelativeStickyBoxOffset.moveBy(
+        FloatPoint(-enclosingLayerOffset) -
+        FloatSize(contentOffsetInCompositingLayer()));
 
     webConstraint.isSticky = true;
     webConstraint.isAnchoredLeft =
         constraints.anchorEdges() &
         StickyPositionScrollingConstraints::AnchorEdgeLeft;
     webConstraint.isAnchoredRight =
         constraints.anchorEdges() &
         StickyPositionScrollingConstraints::AnchorEdgeRight;
     webConstraint.isAnchoredTop =
         constraints.anchorEdges() &
         StickyPositionScrollingConstraints::AnchorEdgeTop;
     webConstraint.isAnchoredBottom =
         constraints.anchorEdges() &
         StickyPositionScrollingConstraints::AnchorEdgeBottom;
     webConstraint.leftOffset = constraints.leftOffset();
     webConstraint.rightOffset = constraints.rightOffset();
     webConstraint.topOffset = constraints.topOffset();
     webConstraint.bottomOffset = constraints.bottomOffset();
     webConstraint.parentRelativeStickyBoxOffset =
-        roundedIntPoint(stickyBoxOffset);
+        roundedIntPoint(parentRelativeStickyBoxOffset);
     webConstraint.scrollContainerRelativeStickyBoxRect =
         enclosingIntRect(constraints.scrollContainerRelativeStickyBoxRect());
     webConstraint.scrollContainerRelativeContainingBlockRect = enclosingIntRect(
         constraints.scrollContainerRelativeContainingBlockRect());
-    // TODO(smcgruer): Copy fields for nested sticky in cc (crbug.com/672710)
+    // TODO(smcgruer): Until http://crbug.com/702229 is fixed, the nearest
+    // sticky layers may not be composited and we may incorrectly end up with
+    // invalid layer IDs.
+    LayoutBoxModelObject* stickyBoxShiftingAncestor =
+        constraints.nearestStickyBoxShiftingStickyBox();
+    if (stickyBoxShiftingAncestor &&
+        stickyBoxShiftingAncestor->layer()->compositedLayerMapping()) {
+      webConstraint.nearestLayerShiftingStickyBox =
+          stickyBoxShiftingAncestor->layer()
+              ->compositedLayerMapping()
+              ->mainGraphicsLayer()
+              ->platformLayer()
+              ->id();
+    }
+    LayoutBoxModelObject* containingBlockShiftingAncestor =
+        constraints.nearestStickyBoxShiftingContainingBlock();
+    if (containingBlockShiftingAncestor &&
+        containingBlockShiftingAncestor->layer()->compositedLayerMapping()) {
+      webConstraint.nearestLayerShiftingContainingBlock =
+          containingBlockShiftingAncestor->layer()
+              ->compositedLayerMapping()
+              ->mainGraphicsLayer()
+              ->platformLayer()
+              ->id();
+    }
   }
 
   m_graphicsLayer->setStickyPositionConstraint(webConstraint);
 }
 
 void CompositedLayerMapping::updateLayerBlendMode(const ComputedStyle& style) {
   setBlendMode(style.blendMode());
 }
 
 void CompositedLayerMapping::updateIsRootForIsolatedGroup() {
@@ skipping 3131 matching lines @@
   } else if (graphicsLayer == m_decorationOutlineLayer.get()) {
     name = "Decoration Layer";
   } else {
     ASSERT_NOT_REACHED();
   }
 
   return name;
 }
 
 }  // namespace blink