How can I remove the empty info window in Calendar?

In Calendar, an empty Info window has appeared and I can't get rid of it.

• Clicking on the x in the top left corner doesn't work
• Neither does Quitting Calendar
• I can drag the window around on the screen. It's not in a fixed position wrt the Calendar window
• With the Calendar window and the stray window covered by a window from a different app, then change the focus to Calendar, both the Calendar window and the stray window are revealed
• In Mission Control, both windows are visible
• I've tried rebooting and safe boot

I'd be grateful for any suggestions - thanks

I'm on Sequoia 15.5 (24F74)

[Re-Titled by Moderator]

Original Title: Stray Info window in Calendar