Can confirm that with Cuphead.
OpenSUSE Leap 15.0, NVIDIA GTX 660 Ti, Driver 418.31.03, Proton 3.16-7, KDE Plasma 5.12
I run the same Kernel with Manjaro, too, and haven't experienced this yet with bspwm, cinnamon or xfce. Will try with i3 later, which WM/DE do you use @magist3r?
kwin 5.12.6
And I have 2 monitors, so actually I'm not alt-tabbing but switching to second monitor.
This issue happens only in fullscreen mode. I enabled virtual desktop in Proton's winecfg and disabled cursor capturing in fullscreen mode as workaround. Windowed mode works for me too.
proton 3.16-7x1 2019-02
I am running Steam on i3, so that might have something to do with it. It seems to happen with every game I've tried via Steam Play. Rivals of Aether, Cuphead, and Stick Fight. This is my first time opening an issue, so let me know if more information is needed.
uname -a: Linux icarus 4.18.3-1-MANJARO #1 SMP PREEMPT Sat Aug 18 12:18:05 UTC 2018 x86_64 GNU/Linux
steam --version: Running Steam on manjarolinux 17.1.12 64-bit
STEAM_RUNTIME is enabled automatically
Pins up-to-date!
I am using Intel's integrated graphics along with the latest mesa driver, according to Steam.