Your system information

Operating System Version:
Ubuntu 18.04.3 LTS (64 bit)
Kernel Name: Linux
Kernel Version: 5.0.0-32-generic
X Server Vendor: The X.Org Foundation
X Server Release: 12004000
X Window Manager: KWin
Steam Runtime Version: steam-runtime_0.20191007.0

• Steam client version (build number or date): steam-runtime_0.20191007.0
• Distribution (e.g. Ubuntu): Ubuntu 18.04.3 LTS (64 bit)
  • Video Card Driver: X.Org AMD Radeon RX 5700 XT (NAVI10, DRM 3.33.0, 5.0.0-32-generic, LLVM 9.0.0)
  • Video Card Driver Version: 4.5 (Compatibility Profile) Mesa 19.2.0-devel
• Opted into Steam client beta?: [Yes/No] No
• Have you checked for system updates?: [Yes/No] Yes

Please describe your issue in as much detail as possible:

Steam breaks the window composition on startup. Turning on any game via proton dissolves the issue until the next startup. Games works fine through steam play – no other issues experienced outside of it.

(telegram desktop included to show that the issue is affecting other programs and composition as well – ignore the image, it is just a meme)

What I've tried so far:

1. Turning off the steam play (it made everything worse since I couldn't use the workaround to fix it)
2. Disabling GPU accelerated rendering
3. purging & reinstalling steam
4. Restarting/tweaking kwin

Steps for reproducing this issue:

1. Turn of steam