Handle nested position:sticky elements correctly (compositor)

third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h

Index: third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h
diff --git a/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h b/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h
index 84dab3cc2f76ad7170d18bed9235375e5dc16c62..cc60c139b029dac6e5a511591c5fb383ff0c21b4 100644
--- a/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h
+++ b/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h
@@ -113,6 +113,12 @@ class StickyPositionScrollingConstraints final {
     return m_totalContainingBlockStickyOffset;
   }
 
+  const LayoutBoxModelObject* nearestStickyAncestor() const {
+    return m_nearestStickyBoxShiftingStickyBox

[chrishtr, 2017/03/20 17:24:52]

It's not obvious to me why this is correct. Could you explain/add a comment
saying
why?

[smcgruer, 2017/03/21 13:54:54]

Done.

+               ? m_nearestStickyBoxShiftingStickyBox
+               : m_nearestStickyBoxShiftingContainingBlock;
+  }
+
   bool operator==(const StickyPositionScrollingConstraints& other) const {
     return m_leftOffset == other.m_leftOffset &&
            m_rightOffset == other.m_rightOffset &&