Page content slightly transparent, reveals content from another window below

[Jani Uusitalo]

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/

ProblemType: Bug
DistroRelease: Ubuntu 19.04
Package: firefox 64.0+build3-0ubuntu1
ProcVersionSignature: Ubuntu 4.19.0-9.10-generic 4.19.8
Uname: Linux 4.19.0-9-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.20.10-0ubuntu14
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: jani 1748 F.... pulseaudio
BuildID: 20181207223303
Channel: Unavailable
CurrentDesktop: ubuntu:GNOME
Date: Sun Dec 16 15:30:58 2018
Extensions: extensions.sqlite corrupt or missing
ForcedLayersAccel: False
IfupdownConfig:
 # interfaces(5) file used by ifup(8) and ifdown(8)
 auto lo
 iface lo inet loopback
IncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
InstallationDate: Installed on 2018-09-30 (76 days ago)
InstallationMedia: Ubuntu 18.10 "Cosmic Cuttlefish" - Beta amd64 (20180930)
IpRoute:
 default via 10.0.2.2 dev enp0s3 proto dhcp metric 100
 10.0.2.0/24 dev enp0s3 proto kernel scope link src 10.0.2.15 metric 100
 169.254.0.0/16 dev enp0s3 scope link metric 1000
Locales: extensions.sqlite corrupt or missing
PrefSources: prefs.js
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fi_FI.UTF-8
 SHELL=/bin/bash
Profiles: Profile0 (Default) - LastVersion=64.0/20181207223303
RunningIncompatibleAddons: False
SourcePackage: firefox
Themes: extensions.sqlite corrupt or missing
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 12/01/2006
dmi.bios.vendor: innotek GmbH
dmi.bios.version: VirtualBox
dmi.board.name: VirtualBox
dmi.board.vendor: Oracle Corporation
dmi.board.version: 1.2
dmi.chassis.type: 1
dmi.chassis.vendor: Oracle Corporation
dmi.modalias: dmi:bvninnotekGmbH:bvrVirtualBox:bd12/01/2006:svninnotekGmbH:pnVirtualBox:pvr1.2:rvnOracleCorporation:rnVirtualBox:rvr1.2:cvnOracleCorporation:ct1:cvr:
dmi.product.family: Virtual Machine
dmi.product.name: VirtualBox
dmi.product.version: 1.2
dmi.sys.vendor: innotek GmbH