User blog comment:Rappy 4187/Technical Update: November 30, 2016/@comment-5956954-20161130212735/@comment-126761-20161130213808

Essentially the 'z-index' of the popup ended up too low, which meant it showed up _below_ the edit area. Not ideal! (The effect was noticeable on some wikis which have transparency, where you could see it popping up below everything else.)