Under some circumstances, content from one Firefox window hidden behind another leaks onto the top one. Reproducible in both Bionic (with Intel graphics) and Disco (in a VirtualBox VM), reporting this from Disco.

== Steps to reproduce ==
1. Open one Firefox window, drag it to the right side of screen to fill the right half, and navigate to https://www.twitch.tv/rifftrax
2. Open another Firefox window, maximize it on top of the first one, and use this topmost window to open https://www.youtube.com/watch?v=P5dxd-ocaE8 and expand the Youtube view to ’theatre’ mode

== What happens ==
White parts of content in Firefox window #1 cast a spectral shadow on the black parts of content in Firefox window #2. It’s quite subtle, but luckily can be caught in screenshots: see attachments below. Pausing the Youtube video seems to make the effect to go away (the transparency disappears).

== What I’d expect to happen ==
For the topmost window to be completely opaque, i.e. not to see anything from behind the topmost window whether the video is playing or not.

== Other info ==
Though I don’t think this is tied the two sites I’ve used as an example, they’re the first and only ones I’ve happened to encounter this with. I’ve yet to find other reports about this, apart from one /r/Fedora thread mentioning something similar: https://www.reddit.com/r/Fedora/comments/7m9m4h/youtube_videos_are_transparent_kind_of_in_firefox/

Untouched screenshot of top window revealing content from another window below

Same screenshot with added hints about see-through content