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

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

Index: third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
index a41e2de4ccff295e86aa1a0ed7ba473e266f4560..041cafced2d95eb15fdf8bb3232ecda68b6a07d1 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
@@ -268,9 +268,12 @@ PaintLayerPainter::PaintResult PaintLayerPainter::paintLayerContents(GraphicsCon
         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;