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

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

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Simon Hausmann <hausmann@kde.org>
  *                     2000 Stefan Schimanski <1Stein@gmx.de>
  *                     2001 George Staikos <staikos@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2005 Alexey Proskuryakov <ap@nypop.com>
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
@@ skipping 608 matching lines @@
 
     // Paint starting at the nearest self painting layer, clipped to the object itself.
     // TODO(pdr): This will also paint the content behind the object if the object contains
     // 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();
     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);
-        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();
 }
 
 PassOwnPtr<DragImage> LocalFrame::dragImageForSelection(float opacity)
 {
     if (!selection().isRange())
         return nullptr;
 
     m_view->updateAllLifecyclePhasesExceptPaint();
@@ skipping 165 matching lines @@
 {
     m_frame->disableNavigation();
 }
 
 FrameNavigationDisabler::~FrameNavigationDisabler()
 {
     m_frame->enableNavigation();
 }
 
 } // namespace blink