After clicking on "_Edit Text In External Editor_" button, the button text changes to "_watching..._" , but after closing the external editor, the text doesn't change back to "_Edit Text In External Editor_".
Here is a screenshot:

Also I would like to work on it.
@PackElend
please go ahead
It works as expected as we can't tell when a file has been closed in an editor (they aren't all modal editors).
can't you watch local processes and listen to the one called by Joplin?
No, there are too many edge cases:
Most editors aren't modal, so even if the file is closed, the process might still be open
Some editors (like Sublime) starts a process to open the file, then open a second process and close the first one, so we wouldn't know what the second process is. And it doesn't matter what it is anyway because it won't tell you whether the file is still open or not.
I've tried and other contributors tried to solve the problem too, but it can't be solved reliably, so now the app only reports what it can - that it is watching a file and that's all we know. Maybe the file is still being edited, maybe not.
Most helpful comment
No, there are too many edge cases:
Most editors aren't modal, so even if the file is closed, the process might still be open
Some editors (like Sublime) starts a process to open the file, then open a second process and close the first one, so we wouldn't know what the second process is. And it doesn't matter what it is anyway because it won't tell you whether the file is still open or not.
I've tried and other contributors tried to solve the problem too, but it can't be solved reliably, so now the app only reports what it can - that it is watching a file and that's all we know. Maybe the file is still being edited, maybe not.