Don't be overwhelmed. By refactoring the "write xmp logic" i just meant copy'n'paste the existing logic into a new class implementing SimpleCommand and bind the disabled-property of the button to the executable-property of the SimpleCommand.

Thanks @calixtus for your suggestion and apologies for the delayed reply. Using SimpleCommand and binding the properties looks certainly to be an interesting approach. I had a look at the code to understand SimpleCommand a bit more, however it wasn't clear to me how would it fit with BackgroundTask? Currently, writeMetadataToPdf creates a BackgroundTask and passes it to the taskExecutor, if we implement a SimpleCommand that does the same, then couldn't SimpleCommand's "execute" method return before the writing to PDF is actually complete, since writing to PDF is done as a BackgroundTask which could be on a different thread? i.e. the button might get enabled before writing to PDF is actually complete?

Or, shall the SimpleCommand be created first by moving the code from the BackgroundTask's runnable to SimpleCommand.execute, then bind SimpleCommand to the button's disabled state, then create a BackgroundTask something like this: BackgroundTask.wrap(()->simpleCommand.execute())?

Unfortunately I don't seem to find a documentation for SimpleCommand so my understanding is be based on going through the code usages for SimpleCommand, which may mean that I missed something, let me know please what do you think.

SimpleCommand is basically a wrapper for every ui action you can imagine.
Just put the backgroundTask into the execute method of a SimpleCommand and use the onRunning or onSuccess methods to update the executable property and bin the property to the disable-property of the button.

I tried this PR. I like that the button becomes grey for ~0,5 seconds, but when I double click, it opens the attached pdf, even though this button is not supposed to open the pdf. Also, when pressing the button multiple times, I get multiple notifications of having successfully written metadata to pdf, even though the button is still grey. Maybe I am not clicking fast enough haha :D

On the plus side: Have not encountered any error message (yet) and the attached PDF was not destroyed either (at least I think so :D) :-)

Thanks for your contribution @melmorsy