Make yourself offline and close the chat window. From settings, disable Sign in to friends when Steam Client starts. "You should be offline next time. Though the chat window will still appear even if you're offline, it's like that since the last update.
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Issue tracker for Linux version of Steam
Yeah...that's the point. THe chat now pops in your face. It's not the only thing that's obnoxious since the last update, on my Linux machines the Store tends to crap out since they enabled HW acceleration and 2/3 of the times it won't load at all. Pretty frustrating they threw this update without proper testing.
I feel like it's more buggy on Linux right now. When I switch workspaces the whole Steam window goes dark unless it's on library, so I started to do almost everything on the browser. Also, HW acceleration has a memory leak for me, it's starting to use my all GPU memory after some time and the fix for that is restarting Steam. Could be Nvidia's fault but still.
Yeah i also noticed a memory drain. Sigh... What a mess.
They should've used their own codebase, not Chromium or Electron. But here we are.