Hello, I've tried searching on the Internet, but I only found about the notification toast or notification sound, etc. I haven't found anything about my problem so I assume it's a bug.

For some reason, when anyone on Steam messages me, the big chat window — where you see and send messages — pops up and I couldn't find a way to disable it anywhere. Disabling notifications or changing any setting in the Notifications tab in Settings doesn't help.
Here's a screenshot of what window I'm talking about:
https://imgur.com/a/ou1mDMq