How to turn off popup notification in private chat?
To turn off the pop-up notification of the private chat, you need to open the chat settings. Click on the gear icon at the bottom of the users’ list.
You can enable or disable the pop-up notification via “Private chat popup.” (beta)

Click the gear icon at the bottom of the users’ list to open chat settings. Find the “Private chat popup” toggle and switch it off.
It is found in the chat settings panel. Click the gear icon at the bottom of the users’ list, then look for “Private chat popup.”
Yes. Go back to the same gear icon, open chat settings, find “Private chat popup,” and toggle it back on at any time.
It shows a pop-up alert when you receive a new private message, even if you are not currently viewing that private chat window.
No. This setting only controls private chat pop-up notifications. Group chat notifications are not affected.