Thonny froze on macOS 27, so I built a flasher and a code editor into this site's Workbench to keep working.
Then I went back, found the bug (the menu click is handled one millisecond too early) and posted what I found upstream.
The tools from the story: Flash a board and the IDE.
Thonny is the Python IDE for beginners by Aivar Annamaa and its contributors. The macOS 27 fix is thonny/thonny#3937.