Correct for enclosing layers scroll position in sticky_data->main_thread_offset

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 cb2b7ed3227dc3d06d6377d2d01149bba06a465f..1c084362a92a347c223e779f7eb94dbd8a33e399 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -308,11 +308,20 @@ void CompositedLayerMapping::updateStickyConstraints(
             &m_owningLayer);
 
     // Find the layout offset of the unshifted sticky box within its enclosing
-    // layer.
+    // layer. If the enclosing layer is not the scroller, the offset must be
+    // adjusted to include the scroll offset to keep it relative.
+    PaintLayer* enclosingLayer =
+        m_owningLayer.enclosingLayerWithCompositedLayerMapping(ExcludeSelf);
     LayoutPoint enclosingLayerOffset;
-    m_owningLayer.enclosingLayerWithCompositedLayerMapping(ExcludeSelf)
-        ->convertToLayerCoords(m_owningLayer.ancestorOverflowLayer(),
-                               enclosingLayerOffset);
+    enclosingLayer->convertToLayerCoords(m_owningLayer.ancestorOverflowLayer(),
+                                         enclosingLayerOffset);
+    if (enclosingLayer != m_owningLayer.ancestorOverflowLayer() &&
+        enclosingLayer->containingLayer()) {
+      enclosingLayerOffset += enclosingLayer->containingLayer()
+                                  ->layoutBox()
+                                  ->scrolledContentOffset();

[flackr, 2017/02/22 14:23:09]

scrolledContentOffset requires that the box has an overflow clip but what if the
containing layer is not a scroller? Is this required if the enclosing layer is a
descendant of the scroller (in which case we can use ancestorOverflowLayer's
scroll offset) or just a child?

[smcgruer, 2017/02/22 15:42:30]

Done.

+    }
+
     FloatPoint stickyBoxOffset =
         constraints.scrollContainerRelativeStickyBoxRect().location();
     DCHECK(!m_contentOffsetInCompositingLayerDirty);