This is very useful - FiniteSingularity is rapidly becoming another Exeldro in the usefulness, capabilities, and applicability of his plugins. Thanks so much for your contributions.
One possible enhancement to this plugin would be to create custom shapes where n number of custom points with their x/y coordinates, opacity, feather width, corner radius, could be defined. (There is an existing polymask plugin which allows 8 'x' and 8 'y' coordinates to be defined which works reasonably well, but it doesn't have corner radius, opacity, feather etc...) It's nice to have regular shapes which can be easily defined, but irregular polygons would save the user from having to create such masks using an external program....
Unfortunately so far I am not aware of any.
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Update:
A potential workaround by achow101 can be found in this GitHub issue:
https://github.com/WarmUpTill/SceneSwitcher/issues/512#issuecomment-2372785305
It introduces a custom condition type "KWin Cursor" via the scripting interface.
Under the hood it uses "kdotool" to query the cursor position.
So, if this tool should be available for your particular Linux distribution it might be worth a shot.