My problem also started after upgrading to Catalina 10.15.1.
- My mail app will not launch the viewer window automatically although the Mail app is open. This is with an initial app launch. If the app is open (once I've gone to Find–>New Viewer Window, and clicked on that), the viewer window will pop up like normal.
- Although Force Quit doesn't indicate a problem, Mail also will not quit without force.
Thanks for your help!