We got the feedback of teachers not using the course chat, because it's not behaving nicely on mobile devices and so they are using external tools for synchronous communication. What I see is that the dialog box has a fixed pixel width and height and I hope the user experience on small screens can be improved by tweaking some of the relevant CSS for a better responsiveness.
On a smart phone the chat dialog should fill the entire screen.
In particular, iOS has a nasty behaviour that zooms OpenOLAT whenever the user inputs something in a form. When the form gets submitted the zoom effect stays which breaks the entire layout. This must be addressed as well