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 60bc7d9c1f0b10387e35cea5af6102cff05cc949..c9f095cb58305cc9f965ca47a6da1a8f1750a032 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -311,11 +311,19 @@ 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()) {
+      enclosingLayerOffset += LayoutSize(enclosingLayer->ancestorOverflowLayer()

[flackr, 2017/02/22 20:44:00]

nit: use ancestorOverflowLayer (from line 296) here and above

[smcgruer, 2017/02/22 20:48:18]

Done.

+                                             ->getScrollableArea()
+                                             ->getScrollOffset());
+    }
+
     FloatPoint stickyBoxOffset =
         constraints.scrollContainerRelativeStickyBoxRect().location();
     DCHECK(!m_contentOffsetInCompositingLayerDirty);