Following up on what others have discovered, there are sometimes when you can’t close the last window and can’t close the file.
Attempting troubleshooting, I opened the View Organizer and one by one closed all of the other files that were open. When down to the last file I looked and the Forms and the Procedures lists for the remaining file and could not open and forms or procedures. I could click on them, but nothing happened. I then opened a few other files and windows, and I could not only open files and procedures from the formerly inaccessible file, I could close it. (in the past I resorted to closing PanX).
Don’t know if this is helpful for diagnosing the problem, and if anyone else can replicate it. Being a random problem, it may be a while before I can try again.