When scaling the desktop to e.g. 150%, fullscreen video does seem to trigger constant changes of the scaling factor: the system switches every 0.5s from 150% to 100% to 200% or things like those.
The problem does not occur, when there is another windows present (e.g. a "always-on-top" terminal windows).
Also, KDE doesn't have this problem.
The problem exists in freshly installed 19.10 and 20.04, too. I couldn't find any reason, why sometimes firefox fullscreen videos do work and sometimes not.
Video playback (e.g. GNOME video player or VLC) does not work properly in 19.10, whereas it works most of the time in 20.04.
How can I help to dig into this?
1) Ubuntu 19.10, 20.04
2) latest available (problem exists since several weeks)
3) What you expected to happen: fullscreen video playback in scaled mode
4) What happened instead: screen starts changing (randomly?) the scaling factor or at least this is the impression one gets
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ProblemType: Bug
ApportVersion: 2.20.11-0ubuntu16
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
DisplayManager: gdm3
DistroRelease: Ubuntu 20.04
InstallationDate: Installed on 2020-01-31 (19 days ago)
InstallationMedia: Kubuntu 20.04 LTS "Focal Fossa" - Alpha amd64 (20200130)
Package: gnome-shell 3.34.3-1ubuntu1
PackageArchitecture: amd64
ProcVersionSignature: Ubuntu 5.4.0-12.15-generic 5.4.8
RelatedPackageVersions: mutter-common 3.34.3-1ubuntu1
Tags: focal
Uname: Linux 5.4.0-12-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip lpadmin lxd plugdev sambashare sudo
_MarkForUpload: True
To make it clear: you are using the GNOME Flashback (Metacity) session, not the default GNOME session, right?