Ah: Number 1 is pretty straightforward, using the viewWillDisappear method, to add a stop command. Still can't find anything about the 2nd question.

OK, I need to change the Scheme, under Product menu, to get the app to build again. I was having a nightmare with build settings, but that seems to have calmed down.

"normally domain should not be nil (you risk a crash in filters)" What should it be? "you get the same result, because of the cast as!." The same result as what? "Result is optional, but cannot be nil, it is forced unwrapped" If it cannot be nil, then why does it need to be optional? And why is it Any, and not 'actual' QuartzFilter? And what about the two different types of property? In your function, the result is a force unwrapped Any -- so why does it return an Optional? I guess this is just a complaint about the way that Apple has chosen to do things, which doesn't really seem to have any benefit, but I suppose I'll have to live with it. More exclamation marks!!!

To add to this, not even shell scripts running as PDF Services, can save files.

I've found a fix until Apple decides to slacken off its evil grip over my computer. ;-) In Automator, make an Application that runs your shell script. Raw scripts, Print plug-ins, workflows or other kinds no longer work as PDF Services. This works for me with my python directly in the Automator "Run Shell Script" action. Whether it will let you pass control to a separate process, I don't know.

My python scripts that run as PDF Services don't work in Big Sur anymore. Console is full of Sandbox deny messages for python Library files. I'm guessing Apple has prevented PDF Services from executing python or other code. "For security reasons". Tsk.