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..57b8c522eae0d0b15831df6be49b3e0006c6d619 100644
--- a/third_party/WebKit/Source/core/frame/LocalFrame.cpp
+++ b/third_party/WebKit/Source/core/frame/LocalFrame.cpp
@@ -626,9 +626,34 @@ PassOwnPtr<DragImage> LocalFrame::nodeImage(Node& node)
     FloatRect boundingBox = layer->layoutObject()->absoluteToLocalQuad(FloatQuad(absoluteBoundingBox), UseTransforms).boundingBox();
     DragImageBuilder dragImageBuilder(this, boundingBox, &node);
     {
-        PaintLayerPaintingInfo paintingInfo(layer, LayoutRect(boundingBox), GlobalPaintFlattenCompositingLayers, LayoutSize(), 0);
-        PaintLayerFlags flags = PaintLayerHaveTransparency | PaintLayerAppliedTransform | PaintLayerUncachedClipRects;
-        PaintLayerPainter(*layer).paintLayer(dragImageBuilder.context(), paintingInfo, flags);
+        // If our enclosing layer is not a stacking context, there could be descendant stacked
+        // contents that are painted by our parent stacking context, i.e. being painting sibling
+        // of our enclosing layer. Collect those descendants here.
+        Vector<PaintLayer*> descendantStackedContents;
+        if (!layer->stackingNode()->isStackingContext()) {
+            for (LayoutObject* curr = layoutObject->nextInPreOrder(layoutObject); curr;) {
+                if (curr->isBoxModelObject() && toLayoutBoxModelObject(curr)->hasSelfPaintingLayer()) {
+                    descendantStackedContents.append(toLayoutBoxModelObject(curr)->layer());
+                    curr = curr->nextInPreOrderAfterChildren(layoutObject);
+                    continue;
+                }
+                curr = curr->nextInPreOrder(layoutObject);
+            }
+            std::stable_sort(descendantStackedContents.begin(), descendantStackedContents.end(),
+                [](PaintLayer* first, PaintLayer* second) { return first->stackingNode()->zIndex() < second->stackingNode()->zIndex(); });
+        }
+
+        PaintLayerPaintingInfo paintingInfo(layer, LayoutRect(boundingBox), GlobalPaintFlattenCompositingLayers, LayoutSize());
+        PaintLayerFlags flags = PaintLayerHaveTransparency | PaintLayerUncachedClipRects;
+
+        auto descendantStackedContentsIterator =  descendantStackedContents.begin();
+        for (; descendantStackedContentsIterator != descendantStackedContents.end() && (*descendantStackedContentsIterator)->stackingNode()->zIndex() < 0; descendantStackedContentsIterator++)
+            PaintLayerPainter(**descendantStackedContentsIterator).paintLayer(dragImageBuilder.context(), paintingInfo, flags);
+
+        PaintLayerPainter(*layer).paintLayer(dragImageBuilder.context(), paintingInfo, flags | PaintLayerAppliedTransform);

[trchen, 2016/05/17 03:27:57]

I felt we should also address pdr's comment on line 621. This line here will
paint other contents that belongs to the same (psuedo) stacking context that are
not necessarily descendants of the target element.
ObjectPainter::paintAllPhasesAtomically can be used but calculating paint offset
is a pain.

+
+        for (; descendantStackedContentsIterator != descendantStackedContents.end(); descendantStackedContentsIterator++)
+            PaintLayerPainter(**descendantStackedContentsIterator).paintLayer(dragImageBuilder.context(), paintingInfo, flags);
     }
     return dragImageBuilder.createImage();
 }