This works, for future reference.

Logging sizes produces these numbers for heights of contentSize, viewSize, myContentSize on several iterations of run, quit, run: window content size 420.0, scroll content size 367.0, view size 305.5; window content size 420.0, scroll content size 367.0, view size 305.5; window content size 420.0, scroll content size 343.0, view size 305.5; window content size 420.0, scroll content size 367.0, view size 305.5 Why the variation?

Just a correction. It's not openURL but NSWorkspace.shared.open that opens the document. It works fine, but, as I said, it's now bound to a file that is not where the user wants (the app is sandboxed, so it's hiding inside the container in the Library folder).