[Bug 1808927] [NEW] On fullscreen mode, video playback is somewhat traslucent in Firefox

[Star Man]

Public bug reported:

Description:

*In Mozilla Firefox browser and when playing HTML5 fullscreen videos, elements on the Gnome 3 desktop (like floating windows) are somewhat still visible (video isn't 100 % opaque).
*The issue occurs only when video is playing, not when paused. Transparency occurs mainly when displaying dark tones (like blues), but not black.
*Tested with Youtube, Dailymotion and Vimeo videos.
*Tested with official Ubuntu repository package and also with Snap package: issue occurs on both of them.
*chromium-browser package from Ubuntu repository also tested: issue isn't present.

System and package details:

uname -a
Linux ***** 4.18.0-12-generic #13-Ubuntu SMP Wed Nov 14 15:17:05 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

lsb_release -rd
Description:	Ubuntu 18.10
Release:	18.10

apt-cache policy firefox
firefox:
  Instalados: 64.0+build3-0ubuntu0.18.10.1
  Candidato:  64.0+build3-0ubuntu0.18.10.1
  Tabla de versión:
 *** 64.0+build3-0ubuntu0.18.10.1 500
        500 http://archive.ubuntu.com/ubuntu cosmic-updates/main amd64 Packages
        500 http://archive.ubuntu.com/ubuntu cosmic-security/main amd64 Packages
        100 /var/lib/dpkg/status
     63.0+build1-0ubuntu1 500
        500 http://archive.ubuntu.com/ubuntu cosmic/main amd64 Packages

Summary:

*What is expected to happen:
Fullscreen HTML5 video should be 100 % opaque (desktop elements shouldn't be visible at all).

*What happened instead:
Desktop elements are subtly still visible under Fullscreen HTML5 video, only when video is playing (not when paused) and mainly with dark tones, not with black.

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: firefox 64.0+build3-0ubuntu0.18.10.1
ProcVersionSignature: Ubuntu 4.18.0-12.13-generic 4.18.17
Uname: Linux 4.18.0-12-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.20.10-0ubuntu13.1
Architecture: amd64
AudioDevicesInUse:
 USER        PID ACCESS COMMAND
 /dev/snd/controlC0:  julian     3065 F.... pulseaudio
BuildID: 20181207224759
Channel: Unavailable
CurrentDesktop: ubuntu:GNOME
Date: Tue Dec 18 02:02:22 2018
EcryptfsInUse: Yes
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 2018-11-15 (32 days ago)
InstallationMedia: Ubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.3)
IpRoute:
 default via 192.168.0.1 dev wlp2s0 proto dhcp metric 600
 169.254.0.0/16 dev wlp2s0 scope link metric 1000
 192.168.0.0/24 dev wlp2s0 proto kernel scope link src 192.168.0.37 metric 600
Locales: extensions.sqlite corrupt or missing
PrefSources: prefs.js
Profiles: Profile0 (Default) - LastVersion=64.0/20181207224759
RunningIncompatibleAddons: False
SourcePackage: firefox
Themes: extensions.sqlite corrupt or missing
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 03/16/2016
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: GL552VW.218
dmi.board.asset.tag: ATN12345678901234567
dmi.board.name: GL552VW
dmi.board.vendor: ASUSTeK COMPUTER INC.
dmi.board.version: 1.0
dmi.chassis.asset.tag: ATN12345678901234567
dmi.chassis.type: 10
dmi.chassis.vendor: ASUSTeK COMPUTER INC.
dmi.chassis.version: 1.0
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvrGL552VW.218:bd03/16/2016:svnASUSTeKCOMPUTERINC.:pnGL552VW:pvr1.0:rvnASUSTeKCOMPUTERINC.:rnGL552VW:rvr1.0:cvnASUSTeKCOMPUTERINC.:ct10:cvr1.0:
dmi.product.family: GL
dmi.product.name: GL552VW
dmi.product.sku: ASUS-NotebookSKU
dmi.product.version: 1.0
dmi.sys.vendor: ASUSTeK COMPUTER INC.

** Affects: firefox (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug cosmic

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Title:
  On fullscreen mode, video playback is somewhat traslucent in Firefox

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