Cleanup: Use foreground rect in paintChildClippingMaskForFragments().

Source/core/paint/LayerPainter.cpp

Index: Source/core/paint/LayerPainter.cpp
diff --git a/Source/core/paint/LayerPainter.cpp b/Source/core/paint/LayerPainter.cpp
index 69a440d5e87ea14b0c6a705be488ad0850c01c0e..bc2964844ef4fd325414a105179d5b12b887456b 100644
--- a/Source/core/paint/LayerPainter.cpp
+++ b/Source/core/paint/LayerPainter.cpp
@@ -822,11 +822,11 @@ void LayerPainter::paintChildClippingMaskForFragments(const LayerFragments& laye
         OwnPtr<ClipRecorder> clipRecorder;
         if (localPaintingInfo.clipToDirtyRect && needsToClip(localPaintingInfo, fragment.foregroundRect)) {
             clipRecorder = adoptPtr(new ClipRecorder(&m_renderLayer, context, DisplayItem::ClipLayerFragmentClippingMask, fragment.foregroundRect));
-            applyRoundedRectClips(localPaintingInfo, context, fragment.foregroundRect, paintFlags, *clipRecorder, IncludeSelfForBorderRadius); // Child clipping mask painting will handle clipping to self.
+            applyRoundedRectClips(localPaintingInfo, context, fragment.foregroundRect, paintFlags, *clipRecorder, IncludeSelfForBorderRadius);
         }
 
         // Paint the the clipped mask.
-        PaintInfo paintInfo(context, pixelSnappedIntRect(fragment.backgroundRect.rect()), PaintPhaseClippingMask, PaintBehaviorNormal, paintingRootForRenderer, 0, localPaintingInfo.rootLayer->renderer());
+        PaintInfo paintInfo(context, pixelSnappedIntRect(fragment.foregroundRect.rect()), PaintPhaseClippingMask, PaintBehaviorNormal, paintingRootForRenderer, 0, localPaintingInfo.rootLayer->renderer());

[chrishtr, 2014/11/06 02:03:04]

The rect passed here is supposed to be the bounds of the renderer. Why is the
foreground rect the right one, not the background rect?

Also, the code on line 825 may not actually execute if there is no clipping
needed.

[mstensho (USE GERRIT), 2014/11/06 14:06:41]

No, it's supposed to be the damage rectangle, isn't it?
"Painting" a "child clipping mask" doesn't seem to really be about actual
painting, but rather about setting a clip mask for children of this layer, for
compositing. See BoxPainter::paintClippingMask(). It just fills the paint area
with black. Since this clipping mask is for children, we want to use the
foreground rect, since the background rect may include a border that children
shouldn't paint on top of.
That's why I think it makes sense to be consistent about which rectangle to use.

[chrishtr, 2014/11/06 18:10:01]

Bear with me, I am learning some of this code also. :)

I see. And the test you added does fail if you revert this line? How does it
fail exactly? I'm trying to see the line of code in BoxPainter that fails. Is it
that it incorrectly early outs?

Also, does Firefox render the test in this CL the same way you expected.

[mstensho (USE GERRIT), 2014/11/06 19:06:08]

No, it didn't fail prior to my changes. Like it says in the description, it
WOULD fail if I had changed everything (i.e. the clipping code above) to using
the background rect. I tried to change everything to using the background rect,
just to see if any existing test would fail then, but I couldn't find any, so I
thought it was best to add something that would fail.

No behavioral changes are intended with this change; it's just about being
consistent (so that I can land.https://codereview.chromium.org/704753002/ )
Firefox renders it correctly too.

         m_renderLayer.renderer()->paint(paintInfo, toPoint(fragment.layerBounds.location() - m_renderLayer.renderBoxLocation() + subPixelAccumulationIfNeeded(localPaintingInfo.subPixelAccumulation, m_renderLayer.compositingState())));
     }
 }