ScreenClipping
if (ScreenClipping(windowContentClipping).Contains(where))
BRegion redraw(ScreenClipping(windowContentClipping));
effectiveClipping->Exclude(&ScreenClipping(windowContentClipping));
BRegion& ScreenClipping(BRegion* windowContentClipping,
_MarkContentDirty(&layer->ScreenClipping(&fContentRegion));
effectiveClipping.IntersectWith(&layer->ScreenClipping(&fContentRegion));
effectiveClipping.IntersectWith(&layer->ScreenClipping(&fContentRegion));