There overflow: hidden; on one of the its parents, so you'd assume that it shouldn't blur outside of the parent container. I'd call that a bug.

[Spec](http://dev.w3.org/fxtf/filters/#filter-region-for-shorthands) hints at this being expected behavior:

“Furthermore, if a shorthand filter expands this visible area like it is the case for <blur()> or <drop-shadow()> the filter region must cover this area as well.”

Firefox behaves the same way AFAICT. http://i.imgur.com/YCygCb7.png