Hello @GoldenAngel1997, in general, no OpenGL application including Steam should be capable of taking down the whole system, which hints that you've encountered a video driver issue.
@kisak-valve Would it be best, to perform re-install, of the initial/existing driver? Or to engage full removal/install? [Keep using current, or dial back to next older driver set?]. Never encountered this level of trouble, for otherwise fresh run of software/drivers
Unfortunately, I don't have any guidance to suggest on the system side, but I have a hunch that the issue you're seeing is a video driver issue that is being aggravated by libcef.
Maybe try running steam steam://open/minigameslist in a terminal. This will tell Steam to start in "Small mode" which does not use libcef to render.
If that's successful, then go to Steam -> Settings -> Interface and uncheck Enable GPU accelerated rendering in web views and if you're lucky, that might avoid the issue when running steam normally.
You'll most likely also want to go to Steam -> Settings -> Library and check Low Performance Mode.
@kisak-valve Much appreciation, for your inputs, through this [unnecessary] ordeal. It did take an awful lot of fumbling / full stop resets, but ultimately got it working (force holding to 435 suite)., without ANY hitch. I did want to give nVIDIA a chance, hoping they've [remotely] softened their attitude with open source, with times advance... but well, healing wounds are still an open wound [insert Linus Torvaldis' vocal tirade].
Nothing extracted yet.
Startup Updater April 09 2019 [found off current bootstrap file]
Kubuntu 20.04.0
Forgone Beta testing
Current Drivers/Headers: nVIDIA Driver 440; Header 5.4.0-42; Up-to-date
Becoming a long-running story, including with multiple steam reinstalls / reflash entire OS. Always happening, after the 1st update gets applied [size ranging from 100MB-220MB in size] / verify update and inputting login info, has the entire GUI freeze to then blackout/crash [no error / warning message at any point] [2-3 cores stuck on 100% load].
Repeatable, by just logging in