Each time I execute my Mp3tag actions list, I always need to undo and redo the actions (sometimes several times) to visually see what changed exactly between when the file is loaded for the very first time and after I executed all actions, or a tag import action.
When you import the tags from a database, sometime some characters are removed (punctuation for example) and this is not very obvious, so I need to manually check tag by tag, file by file...
This is a well-known feature for developers (git diff is a huge time saver) and it would be a killer feature to have a tool like this in Mp3tag.
My idea is simple : When a file is loaded, all its tags are kept in the diff comparator tool waiting for any changes. When a change has been detected, a comparison window could be open/used to see the differences. It could be available from the toolbar for example.
That could work for a small amount of files and changes.
I'm not sure if you want to see and check the changes for several thousands files one by one.
In your example, most changes are just case spelling for single characters. I would never want to check such changes (delivered from online sources). Because you never get it right due to official spelling rules, your own preferences and the artist intent.
I suggest that you test your actions with most (if not all) of your existing cases. Once you are sure that they do what you want, I would not check the results file by file and change by change anymore. You could test some random samples to be sure that no unwanted changes have been made.
And as always: Create backups of your files before you execute your actions.
I am not quite sure which data the diff-window should show.
I have a number of action groups that execute several actions on different fields - and even perform several iterations (like replacing the various incarnations of "featuring" with a standardized string). They even rename files.
So what should be shown in which way?
Your example shows the contents of 1 field - how should it behave for more or less all the other fields?
I would suggest for the meantime that you keep your own watch with a suitable export to a text file of some sort and then run an external program for the comparison.
Also, if you do not trust your actions, then I would suggest that you use smaller steps, dedicated filters to narrow down the files that you really want to treat and then process them with very simple actions so that you can be sure that only those changes happened that you intended.
I am not really sure that an optical scan even with highlighted changes would let you find all the dubious cases - to easily one skips over such things if there have been a lot of them. And if you do it that thoroughly that you don't miss anything, then you could have changed all the cases manually in the same amount of time.
I do not really want to argue against such a function, I only doubt that it would really be such a leap forward.
If there is a usecase for a comparison then MP3tag already today offers the interface to let other programs (and probably better and proven ones) do the job.