Calculate correct clip foregroundBound and backgroundBound

Source/core/paint/DeprecatedPaintLayerClipper.cpp

Index: Source/core/paint/DeprecatedPaintLayerClipper.cpp
diff --git a/Source/core/paint/DeprecatedPaintLayerClipper.cpp b/Source/core/paint/DeprecatedPaintLayerClipper.cpp
index 4545e9497330e06859daee4296b23cb88f085714..aebd5df81860a32cce8a6456dc3795f73cae5c7c 100644
--- a/Source/core/paint/DeprecatedPaintLayerClipper.cpp
+++ b/Source/core/paint/DeprecatedPaintLayerClipper.cpp
@@ -217,22 +217,23 @@ void DeprecatedPaintLayerClipper::calculateRects(const ClipRectsContext& context
 
     if (!isClippingRoot && m_layoutObject.layer()->parent()) {
         backgroundRect = backgroundClipRect(context);
-        backgroundRect.move(roundedIntSize(context.subPixelAccumulation));

[pdr., 2015/08/17 21:17:53]

We used roundedIntSize before, but don't now. Was that intentional or tested?

[qiankun, 2015/08/19 09:14:19]

It's intentional. The layout test which was added for this line passed. 

         backgroundRect.intersect(paintDirtyRect);

[pdr., 2015/08/17 21:17:53]

This seems wrong... we're intersecting with the paintDirtyRect and then moving
by the subPixelAccumulation below. I think we need to do dirty rect intersection
before moving.

[qiankun, 2015/08/19 09:14:19]

Both backgroundRect and paintDirtyRect should move by subPixelAccumulation here.
But backgroundRect and paintDirtyRect didn't consider subPixelAccumulation, so
moving subPixelAccumulation after intersection.

[pdr., 2015/08/20 05:55:13]

I'm just worried about the order these are being done.

For example (x, y, w, h):
paintDirtyRect: (0, 0, 100, 100)
backgroundRect: (-1, 0, 1, 100)
subpixelAccumulation: (0.5, 0)

Without your patch:
backgroundRect.move(subpixelAccumulation) = (-0.5, 0, 1, 100)
backgroundRect.intersect(paintDirtyRect) = (0, 0, 0.5, 100)
This will paint a line at (0, 0) with width 0.5px and height 100px.

With your patch:
backgroundRect.intersect(paintDirtyRect) = (0, 0, 0, 0)
backgroundRect.move(subpixelAccumulation) = (0.5, 0, 0, 0)
This will paint nothing because it's empty.

[qiankun, 2015/08/20 06:08:43]

Current implementation is wrong here. I think we should also do
paintDirtyRect.move(subpixelAccumulation) before intersection. 

     } else {
         backgroundRect = paintDirtyRect;
     }
 
-    foregroundRect = backgroundRect;
-    outlineRect = backgroundRect;
-
     LayoutPoint offset;
-    if (offsetFromRoot)
+    if (offsetFromRoot) {
         offset = *offsetFromRoot;
-    else
+        backgroundRect.move(context.subPixelAccumulation);
+    } else {
         m_layoutObject.layer()->convertToLayerCoords(context.rootLayer, offset);
+    }
     layerBounds = LayoutRect(offset, LayoutSize(m_layoutObject.layer()->size()));
 
+    foregroundRect = backgroundRect;
+    outlineRect = backgroundRect;
+
     // Update the clip rects that will be passed to child layers.
     if (m_layoutObject.hasOverflowClip()) {
         // This layer establishes a clip of some kind.