MXGP3 (561600) fullscreen mode is not full screen

Compatibility Report

• Name of the game with compatibility issues: MXGP3
• Steam AppID of the game: 561600

System Information

• GPU: RX Vega 64

• Driver/LLVM version: mesa 19.0.4/8.0.0

• Kernel version: 5.1.1

• Link to full system information report as Gist: https://gist.github.com/amezin/27122ea029f8fa0b6b979d5ca93c9c1e

• Proton version: 4.2-3

I confirm:

• that I haven't found an existing compatibility report for this game.
• that I have checked whether there are updates for my system available.

steam-561600.log

Symptoms

In fullscreen mode the game occupies only a quarter of the screen, when game resolution is set to 1920x1080 and screen resolution is 3840x2160

Reproduction

• Have a 4k monitor
• Launch the game
• Options->Video->Resolution: 1920x1080
• Options->Video->In Window: no

This game has a native port, but it is almost unplayable because of graphical glitches, so I run the game using Proton.

Start by reproducing MXGP3 with a 4K display, 1920x1080 game resolution, and fullscreen enabled, then inspect the linked steam-561600.log. Done means the game fills the display at the selected resolution instead of occupying only a quarter of the screen.