/set lafCustomTheme windowDecoration off
Restart Chatty and window management should now be working, but the title bar will either be white or a system colour, so let's fix that.
Open your start menu and search for "accent color".
Select the "Choose your accent color" result.
Tick the "Title bars and window borders" option at the bottom, then close the window.
Finally, run this reg file to fix the title bar colours and text size: chatty.reg
You will then need to logout and log back in (or reboot) for this change to apply.