Extending it didn't play well with invalidation; widgets would only be able to
draw outside if the invalidated region of the current paint was larger than the
widget's declared paint rect but not if the widget was the only thing that got
invalidated.
Any legitimate widget overflow should instead be handled by GetWidgetOverflow.
The DrawCellWithSnapping overflow is considered to stay within the focus
ring's bounds.
I fuzzed two reftests which have extremely slight variance when -moz-appearance
is combined with box-shadow. I don't really understand this failure but I don't
think it's worth looking into either.
MozReview-Commit-ID: ECYxnCTafdh
a367ab3542