Once bug 914617 is implemented and performance of that is acceptable, we can backout bug 921038 and bug 933674 to switch back to using the clip-path property of CSS to get proper pointer events on the selected tab when lightweight themes (LWT) aren't applied. I personally think we can ship as-is so I'm marking as a non-blocking P4. If bug 914617 comes through, it should only be a matter of backing out the two bugs so I'd still do it if we have time.