Same issue here:

The first time it happened I restarted the client and it was fixed but now it's happened again and restarts aren't fixing it.
I can confirm this bug in the clients chat. This doesn't happen in the webchat.
Your system information
This happens in the beta client for quite some while, even in Steam Overlay this happens. However, it's random for me and closing the chat window and open it again seems to fix it once.
Can confirm, ubuntu 18. I don't think I opted into the beta.
Can confirm - sometimes closing and reopening it helps, but more often than not the black edge+shift stays
Steam client version (build number or date): 1565407153 / Aug 10 2019 um 02:00:27
Distribution (e.g. Ubuntu): Manjaro KDE
Opted into Steam client beta?: Yes
Have you checked for system updates?:Yes
edit: 1565473496 still hast the same issue
Confirmed under Manjaro Cinnamon with Steam build 1565057304, too.
Experiencing this on Debian as well, and a friend of mine has experienced it too on Windows with the in-game overlay, so it doesn't seem to be a Linux-specific issue
Why is this closed? I had this problem in the ingame chat overlay today.
Edit: Got this problem yesterday in the normal chat.
Same, seems not to happen on the desktop anymore, but just happened in the overlay on me again.
Can anyone still confirm this bug? I haven't had this for a long time now.
I too haven't had it long enough that i actually forgot about it, would consider it fixed.
Since no one posted here that he still has this bug I'm going to close it for now.
Nothing extracted yet.
Your system information
Please describe your issue in as much detail as possible:
Since the latest update the chat is shifted to the left. I can't see the cursor anymore and instead I've some black space on the right. See screenshot. I already restarted my system but with no success. I also tried minimizing/maximizing/resize the chat window with no success either.
Steps for reproducing this issue: