Your system information

• Steam client version (build number or date): 1603487799 / Oct 23 2020, at 20:26:58
• Distribution (e.g. Ubuntu): Ubuntu (also tested on Kubuntu)
• Opted into Steam client beta?: No
• Have you checked for system updates?: Yes

Please describe your issue in as much detail as possible:

Exclusively with Steam games, if I have mismatching monitors (resolution), or intentionally set a monitor to a different resolution than the primary monitor; a full-screen game can launch on one monitor but with the resolution of the other. (I have not discovered what determines which way round it is.)

For example, primary monitor is set to 800x600, and game goes full-screen on the 1080p monitor at 800x600.
If I reverse the problem, then I'll have a game that launches at 1080p, but on the 800x600 monitor.

This problem was discovered by @Joestone13 running Kubuntu with a 1080p and 1440p monitor where games used the wrong resolution, and reproduced by myself running Ubuntu with a 1080p monitor, and a 1080p monitor set to 800x600 in display settings.

This has been verified with the following titles on Steam:

• PAYDAY 2
• 10,000,000

I have tested a non-Steam game called osu!, and the browser, Firefox to see if similar issues occur when fullscreening but it does not.

Steps for reproducing this issue:

1. Set your primary monitor, and other monitors to different resolutions.
2. Launch a game into full screen.