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..dd4da51ec886ddd81f925d987bc92a2790eae0f9 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()) {

[flackr, 2017/02/22 15:45:18]

Why the check for containingLayer? If it's not the ancestorOverflowLayer it will
for sure have a containing layer right?

[smcgruer, 2017/02/22 15:58:23]

Ah, yes, no longer needed.

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