Compute better reference/visual boxes for clip-path in columns

third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/PaintLayerPainter.h"
 
 #include "core/frame/LocalFrame.h"
 #include "core/layout/LayoutInline.h"
 #include "core/layout/LayoutView.h"
 #include "core/paint/ClipPathClipper.h"
@@ skipping 250 matching lines @@
     // These helpers output clip and compositing operations using a RAII pattern. Stack-allocated-varibles are destructed in the reverse order of construction,
     // so they are nested properly.
     Optional<ClipPathClipper> clipPathClipper;
     // Clip-path, like border radius, must not be applied to the contents of a composited-scrolling container.
     // It must, however, still be applied to the mask layer, so that the compositor can properly mask the
     // scrolling contents and scrollbars.
     if (m_paintLayer.layoutObject()->hasClipPath() && (!m_paintLayer.needsCompositedScrolling() || (paintFlags & PaintLayerPaintingChildClippingMaskPhase))) {
         paintingInfo.ancestorHasClipPathClipping = true;
 
         LayoutRect referenceBox(m_paintLayer.boxForClipPath());
-        referenceBox.moveBy(offsetFromRoot);
+        if (m_paintLayer.enclosingPaginationLayer())
+            m_paintLayer.convertFromFlowThreadToVisualBoundingBoxInAncestor(paintingInfo.rootLayer, referenceBox);

[mstensho (USE GERRIT), 2016/09/02 19:06:01]

This isn't going to work correctly if the clipping path crosses column
boundaries, right? You'll then slice the clip box into multiple fragments and
use the union of those fragments as referenceBox.

I'd expect to see no red here, for instance.

<div style="columns:2; width:300px; background:yellow;">
    <div style="height:300px; border:5px solid red;
-webkit-clip-path:inset(5px); background:lime;"></div>
</div>

I guess the only way to fix this is to apply clip-path AFTER fragmentation.
Could you add a note about this?

[fs, 2016/09/02 19:16:51]

Yeah, we have similar issues with inlines spanning multiple lines. 
Added note.

+        else
+            referenceBox.moveBy(offsetFromRoot);
         clipPathClipper.emplace(
-            context, *m_paintLayer.layoutObject(), FloatRect(referenceBox), FloatPoint(offsetFromRoot));
+            context, *m_paintLayer.layoutObject(), FloatRect(referenceBox), FloatPoint(referenceBox.location()));
     }
 
     Optional<CompositingRecorder> compositingRecorder;
     // Blending operations must be performed only with the nearest ancestor stacking context.
     // Note that there is no need to composite if we're painting the root.
     // FIXME: this should be unified further into PaintLayer::paintsWithTransparency().
     bool shouldCompositeForBlendMode = (!m_paintLayer.layoutObject()->isDocumentElement() || m_paintLayer.layoutObject()->isSVGRoot()) && m_paintLayer.stackingNode()->isStackingContext() && m_paintLayer.hasNonIsolatedDescendantWithBlendMode();
     if (shouldCompositeForBlendMode || m_paintLayer.paintsWithTransparency(paintingInfo.getGlobalPaintFlags())) {
         FloatRect compositingBounds = FloatRect(m_paintLayer.paintingExtent(paintingInfo.rootLayer, paintingInfo.subPixelAccumulation, paintingInfo.getGlobalPaintFlags()));
         compositingRecorder.emplace(context, *m_paintLayer.layoutObject(),
@@ skipping 501 matching lines @@
     if (!m_paintLayer.containsDirtyOverlayScrollbars())
         return;
 
     PaintLayerPaintingInfo paintingInfo(&m_paintLayer, LayoutRect(enclosingIntRect(damageRect)), paintFlags, LayoutSize());
     paintLayer(context, paintingInfo, PaintLayerPaintingOverlayScrollbars);
 
     m_paintLayer.setContainsDirtyOverlayScrollbars(false);
 }
 
 } // namespace blink