Enable position:sticky for <thead> and <tr> 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 33672ccc2d653469824876d32a0794e44c97fdf4..dd2c47d7dbe0e0167b23642b8f2edd94ef13e93d 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
@@ -33,6 +33,7 @@
 #include "core/layout/LayoutFlexibleBox.h"
 #include "core/layout/LayoutGeometryMap.h"
 #include "core/layout/LayoutInline.h"
+#include "core/layout/LayoutTableCell.h"
 #include "core/layout/LayoutView.h"
 #include "core/layout/compositing/CompositedLayerMapping.h"
 #include "core/layout/compositing/PaintLayerCompositor.h"
@@ -981,10 +982,18 @@ void LayoutBoxModelObject::updateStickyPositionConstraints() const {
   // nested position:sticky elements between themselves and their
   // containingBlock, and between the containingBlock and their scrollAncestor.
   //
-  // The respective search ranges are [container, containingBlock) and
+  // The respective search ranges are [locationContainer, containingBlock) and
   // [containingBlock, scrollAncestor).
+
+  // Table cells are a special case; Blink ignores them for locationContainer()
+  // purposes, but their sticky offset does affect descendant sticky objects.
+  // TODO(smcgruer): Remove cell override once Blink supports the correct
+  // locationContainer for CSS 3.
+  LayoutObject* locationContainerForSticky =

[smcgruer, 2017/03/29 18:02:57]

1. I'm not sure if we also want to do this for its use in
skippedContainersOffset above. Thoughts?

2. I thought we would also want to do this for findFirstStickyBetween, but was
unable to construct a test case. I was thinking of something like the following,
thoughts?

<tr class=".sticky">
  <th><div class=".sticky"></div></th>
</tr>

I think in this case the <div> would fail to notice the <tr> sticky ancestor,
but in my tests this doesn't seem to actually cause any problems which is
strange.

+      isTableCell() ? toLayoutTableCell(this)->row() : locationContainer;
+
   constraints.setNearestStickyBoxShiftingStickyBox(
-      findFirstStickyBetween(locationContainer, containingBlock));
+      findFirstStickyBetween(locationContainerForSticky, containingBlock));
   // We cannot use |scrollAncestor| here as it disregards the root
   // ancestorOverflowLayer(), which we should include.
   constraints.setNearestStickyBoxShiftingContainingBlock(findFirstStickyBetween(