Handle nested position:sticky elements

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

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
index 7a0159a8b36d08f32e2bcdd9fb541b1edf20374b..1947d01a4f762cbfabd0d76e17b456a5520e342e 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -286,6 +286,31 @@ void CompositedLayerMapping::updateBackdropFilters(const ComputedStyle& style) {
       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;
@@ -341,6 +366,12 @@ void CompositedLayerMapping::updateStickyConstraints(
         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);