The minimize, expand, close buttons do not seem to be working

What I did: Opened Agenda and then tried to minimize the window

What happened: The red, yellow, and green buttons are visible if I put the arrow just below them, but when I try to click on them they disappear. The menu commands for minimizing, etc. still work.

What I expected:

Things that might be helpful to know (Agenda version, OS and model, etc): Agenda version 11.2.1, OS 11.1, iMac (Retina 4K, 21.5-inch, Late 2015)

See Agenda 11.2.1 doesn’t show close/minimise buttons in macOS 11.1