Currently, if you have some tags in the 'Preserved Tags' list, the comparison won't indicate any changes, because the current logic is to "show what will be changed after you press save". I'd like to request a setting in options (it could also be a plugin), so the logic would change to "show what is different from mb data", even for fields which are currently in the 'Preserved Tags' list. Even better would be to add some kind of visual indicator, like a small icon with more information in tool-tip on hover (see the attached screenshots).

My current motive is that I use "Playback Statistics" plugin with Foobar2000 which stores the individual statistics by looking at specific tags (artist + album + disc number + track number + track title). This is the default method for the plugin which has its advantages (statistics are kept if you change the format from lossy to lossless, if you have multiple files with same tags and if you move files around within your music library). The disadvantage is that changing these tags outside Foobar will reset these statistics, so therefore I need to have these tags in the 'Preserved Tags' list in Picard to keep my statistics. However, in order to figure out if any of these tags differ, I have to remove them from the 'Preserved Tags' list to see if any of them are different and then add them back to the list so I can save the rest of the tags with Picard. There might also be different reasons for other people to preserve their tags and still wanting to see the differences (e.g. people just want to add musicbrainz specific tags to their files and not touch any other tags, but still want to see if any data might be incorrect on either side and come back to it later).

Attaching screenshot of a the current behavior as well as my mock-up of how it could look. Additional advantage is that you can see the difference between "matched" and "preserved" tag at first glance. With current behavior, you don't know whether the Artist tag actually matched the mb data or is also in preserved tags from my screenshot of the current behavior.