Handle nested position:sticky elements

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp

Index: third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
index 486a11a43eb50a95648e8c40706d57d7d6d7f796..07f642ccfc50dddbcbd5b733ddc097a85bdf6db2 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
@@ -873,6 +873,41 @@ void LayoutBoxModelObject::updateStickyPositionConstraints() const {
                     toFloatSize(stickyLocation),
                 flippedStickyBoxRect.size()));
 
+  // To correctly compute the offsets, the constraints need to know about any
+  // nested position:sticky between themselves and their containingBlock, and
+  // between the containingBlock and their scrollAncestor.
+  //
+  // The respective search ranges are [parent(), containingBlock) and
+  // [containingBlock, scrollAncestor).
+
+  // TODO(smcgruer): Fold these into the walks to find the containingBlock and
+  // scrollAncestor.
+
+  LayoutObject* maybeStickyAncestor = parent();
+  while (maybeStickyAncestor && maybeStickyAncestor != containingBlock) {
+    if (maybeStickyAncestor->isStickyPositioned()) {
+      constraints.setNearestStickyElementShiftingStickyBox(
+          toLayoutBoxModelObject(maybeStickyAncestor));
+      break;
+    }
+    maybeStickyAncestor = maybeStickyAncestor->parent();
+  }
+
+  // NOTE(smcgruer): We cannot use |scrollAncestor| here as it disregards the
+  // root ancestorOverflowLayer(), which we should include for the purposes of
+  // finding the nearest sticky element that shifts the containing block rect.
+  LayoutObject* ancestorOverflowLayer =
+      layer()->ancestorOverflowLayer()->layoutObject();
+  maybeStickyAncestor = containingBlock;
+  while (maybeStickyAncestor && maybeStickyAncestor != ancestorOverflowLayer) {
+    if (maybeStickyAncestor->isStickyPositioned()) {
+      constraints.setNearestStickyElementShiftingContainingBlock(
+          toLayoutBoxModelObject(maybeStickyAncestor));
+      break;
+    }
+    maybeStickyAncestor = maybeStickyAncestor->parent();

[flackr, 2017/01/24 20:28:48]

Technically we should be following the locationContainer chain. Walking the
parent chain might give us an incorrect ancestor if we have a parent that is
fixed position (which would not be contained by its parent). i.e.

<sticky>
  <fixed>
    <sticky> <!-- would think that its fixed container is shifted by the sticky
position element above it -->

[smcgruer, 2017/01/28 00:55:31]

Done.

+  }
+
   // We skip the right or top sticky offset if there is not enough space to
   // honor both the left/right or top/bottom offsets.
   LayoutUnit horizontalOffsets =
@@ -970,15 +1005,39 @@ LayoutSize LayoutBoxModelObject::stickyPositionOffset() const {
   PaintLayerScrollableArea* scrollableArea =
       ancestorOverflowLayer->getScrollableArea();
 
+  auto it = scrollableArea->stickyConstraintsMap().find(layer());
+
   // The sticky offset is physical, so we can just return the delta computed in
   // absolute coords (though it may be wrong with transforms).
   // TODO: Force compositing input update if we ask for offset with stale
   // compositing inputs.
-  if (!scrollableArea->stickyConstraintsMap().contains(layer()))
+  if (it == scrollableArea->stickyConstraintsMap().end())
     return LayoutSize();
+
+  StickyPositionScrollingConstraints& constraints = it->value;
+
+  FloatSize ancestorStickyBoxOffset;
+  FloatSize ancestorContainingBlockOffset;
+
+  const LayoutBoxModelObject* toContainingBlock =
+      constraints.nearestStickyElementShiftingStickyBox();
+  if (toContainingBlock) {
+    ancestorStickyBoxOffset = scrollableArea->stickyConstraintsMap()
+                                  .get(toContainingBlock->layer())
+                                  .getTotalStickyBoxStickyOffset();
+  }
+
+  const LayoutBoxModelObject* toViewport =
+      constraints.nearestStickyElementShiftingContainingBlock();
+  if (toViewport) {
+    ancestorContainingBlockOffset = scrollableArea->stickyConstraintsMap()
+                                        .get(toViewport->layer())
+                                        .getTotalContainingBlockStickyOffset();
+  }
+
   return LayoutSize(
-      scrollableArea->stickyConstraintsMap().get(layer()).computeStickyOffset(
-          constrainingRect));
+      constraints.computeStickyOffset(constrainingRect, ancestorStickyBoxOffset,
+                                      ancestorContainingBlockOffset));
 }
 
 LayoutPoint LayoutBoxModelObject::adjustedPositionRelativeTo(