Hello there,
It's been a long time since I have last used mp3tag, but a feature I am almost certain I always used was this:
My startup path was for example "d:\albums-to-tag". This would then (as it does now) list all the files contained in the subfolders (e.g. "d:\albums-to-tag\albumtitle\disc 1").
By double-clicking on a track mp3tag would then open the subfolder of that file as its working path. So from starting at "d:\albums-to-tag" it would hop to "d:\albums-to-tag\albumtitle\disc1" as its working directory.
This would save me the time to browse to a subfolder, I would just double click on a track. Please note that I am not trying to open the subfolder in Windows Explorer - I want to use the double click to open it in mp3tag, so that I only look at the tracks contained in that subfolder.
What am I missing? Did this feature never exist and I just imagined using it 10 years ago?
Kind regards,
a confused fan of mp3tag 
