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..39b51de3caff42f42c1d34f568e1b90173a0a65e 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -311,11 +311,18 @@ 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.

[chrishtr, 2017/02/28 19:36:28]

"relative to compositingContainer"

[smcgruer, 2017/03/01 19:58:39]

Done.

+    PaintLayer* enclosingLayer =
+        m_owningLayer.enclosingLayerWithCompositedLayerMapping(ExcludeSelf);

[chrishtr, 2017/02/28 19:36:28]

Please pass compositingContainer to updateStickyConstraints
(it's available in the callsite).

Also, please make updateStickyConstraints a private member
of CompositedLayerMapping.

[smcgruer, 2017/03/01 19:58:40]

Done.

     LayoutPoint enclosingLayerOffset;
-    m_owningLayer.enclosingLayerWithCompositedLayerMapping(ExcludeSelf)
-        ->convertToLayerCoords(m_owningLayer.ancestorOverflowLayer(),
-                               enclosingLayerOffset);
+    enclosingLayer->convertToLayerCoords(ancestorOverflowLayer,
+                                         enclosingLayerOffset);
+    if (enclosingLayer != ancestorOverflowLayer) {
+      enclosingLayerOffset += LayoutSize(
+          ancestorOverflowLayer->getScrollableArea()->getScrollOffset());
+    }
+
     FloatPoint stickyBoxOffset =
         constraints.scrollContainerRelativeStickyBoxRect().location();
     DCHECK(!m_contentOffsetInCompositingLayerDirty);