I have confirmed this is a change of behaviour in Big Sur, and reported it as a bug to Apple (FB8915183). Unfortunately, I cannot work around the bug until I know whether Apple considers this a bug and will fix it, or considers this a breaking change they have decided to make, otherwise if Apple fixes the bug, then it will produce the same misbehaviour in reverse.
The bug happens when:
- A non-user-resizeable window
- with a saved window location
- is resized in code before being displayed
It affects Keyboard Maestro’s Alerts and Prompt For User Input windows.