Drag images should paint descendants that are painting siblings of the dragged element

third_party/WebKit/Source/core/frame/LocalFrame.cpp

Index: third_party/WebKit/Source/core/frame/LocalFrame.cpp
diff --git a/third_party/WebKit/Source/core/frame/LocalFrame.cpp b/third_party/WebKit/Source/core/frame/LocalFrame.cpp
index ab8078288b9368808d1db8f783d5c55c3ac60220..dccb50f1c9a6015666faf2c0367913af7eb63fe8 100644
--- a/third_party/WebKit/Source/core/frame/LocalFrame.cpp
+++ b/third_party/WebKit/Source/core/frame/LocalFrame.cpp
@@ -617,16 +617,18 @@ PassOwnPtr<DragImage> LocalFrame::nodeImage(Node& node)
     if (!layoutObject)
         return nullptr;
 
-    // Paint starting at the nearest self painting layer, clipped to the object itself.
+    // Paint starting at the nearest stacking context, clipped to the object itself.

[chrishtr, 2016/05/19 17:45:16]

I wonder if we should explicitly clip now, to avoid over-painting of ancestors.

[trchen, 2016/05/23 19:37:53]

As discussed offline, it is already clipped.

     // TODO(pdr): This will also paint the content behind the object if the object contains

[chrishtr, 2016/05/19 17:46:52]

I'd change this to a note instead of a TODO. There is now no intention to "fix"
it.

[trchen, 2016/05/23 19:37:53]

Done.

     // transparency but the layer is opaque. We could directly call layoutObject->paint(...)
     // (see ObjectPainter::paintAllPhasesAtomically) but this would skip self-painting children.
-    PaintLayer* layer = layoutObject->enclosingLayer()->enclosingSelfPaintingLayer();
+    PaintLayer* layer = layoutObject->enclosingLayer();
+    if (!layer->stackingNode()->isStackingContext())
+        layer = layer->stackingNode()->ancestorStackingContextNode()->layer();
     IntRect absoluteBoundingBox = layoutObject->absoluteBoundingBoxRectIncludingDescendants();
     FloatRect boundingBox = layer->layoutObject()->absoluteToLocalQuad(FloatQuad(absoluteBoundingBox), UseTransforms).boundingBox();
     DragImageBuilder dragImageBuilder(this, boundingBox, &node);
     {
-        PaintLayerPaintingInfo paintingInfo(layer, LayoutRect(boundingBox), GlobalPaintFlattenCompositingLayers, LayoutSize(), 0);
+        PaintLayerPaintingInfo paintingInfo(layer, LayoutRect(boundingBox), GlobalPaintFlattenCompositingLayers, LayoutSize());
         PaintLayerFlags flags = PaintLayerHaveTransparency | PaintLayerAppliedTransform | PaintLayerUncachedClipRects;
         PaintLayerPainter(*layer).paintLayer(dragImageBuilder.context(), paintingInfo, flags);
     }