[New Multicolumn] Make rounded corners clipping multicol-aware.

Source/core/paint/LayerPainter.h

Index: Source/core/paint/LayerPainter.h
diff --git a/Source/core/paint/LayerPainter.h b/Source/core/paint/LayerPainter.h
index fae2c89d0581be795d36203ec145b04166aca746..efd63ed56f00f5c8e4371c33f7fb2ea49320b292 100644
--- a/Source/core/paint/LayerPainter.h
+++ b/Source/core/paint/LayerPainter.h
@@ -30,7 +30,16 @@ public:
     void paintOverlayScrollbars(GraphicsContext*, const LayoutRect& damageRect, PaintBehavior, RenderObject* paintingRoot = 0);
 
     enum BorderRadiusClippingRule { IncludeSelfForBorderRadius, DoNotIncludeSelfForBorderRadius };
-    static void applyRoundedRectClips(RenderLayer&, const LayerPaintingInfo&, GraphicsContext*, PaintLayerFlags, ClipRecorder&, BorderRadiusClippingRule = IncludeSelfForBorderRadius);
+
+    // Set rounded clip rectangles defined by border radii all the way from the LayerPaintingInfo

[chrishtr, 2014/11/14 23:22:26]

Aside, unrelated to your CL: why are we setting it all the way from the root?
Why is the root not painting its own rounded corners before recursion? Don't
quite get it, do you?

[mstensho (USE GERRIT), 2014/11/14 23:36:58]

Both rectangular and rounded clip rectangles are applied all the way from the
"root" [1] for all layers, I think. The reason has to be that different
descendant layers are affected differently by ancestors' clipping. E.g. an
absolutely positioned box "CHILD1" inside a statically positioned box "PARENT"
with overflow:hidden isn't clipped by "PARENT", while a relatively positioned
"CHILD2" of the same "PARENT" will get clipped by it.

[1] "root" is not root when we transform. Then "root" becomes the transformed
layer.

Note that this it not about actually painting the borders. That's done locally
on every layer. This is only about clipping along the inner edges of the borders
when painting child content.

+    // "root" layer down to the specified layer (or the parent of said layer, in case
+    // BorderRadiusClippingRule says to skip self). fragmentOffset is used for multicol, to specify
+    // the translation required to get from flow thread coordinates to visual coordinates for a
+    // certain column.
+    // FIXME: The BorderRadiusClippingRule parameter is really useless now. If we want to skip self,
+    // why not just supply the parent layer as the first parameter instead?
+    static void applyRoundedRectClips(RenderLayer&, const LayerPaintingInfo&, GraphicsContext*, const LayoutPoint& fragmentOffset, PaintLayerFlags, ClipRecorder&, BorderRadiusClippingRule = IncludeSelfForBorderRadius);
+
     static void beginTransparencyLayers(GraphicsContext*, RenderLayer&, const RenderLayer* rootLayer, const LayoutRect& paintDirtyRect, const LayoutSize& subPixelAccumulation, PaintBehavior);
 
 private: