Please elaborate ... at which moment resp. how often should the "custom field" pick list be updated?
And ... are you of the opinion, that the pick list should contain all user defined tag-fields, at any time?
The program should update it whenever required to keep it always up-to-date.
Yes.
One might instead say that it should contain only the fields of the currently selected tracks, as does e.g. the Extended Tags the dialogue box. But I think most useful would be for it to contain the fields of all the loaded tracks.
Um, no. My custom fields are available for all files and the mp3tag UI where required (e.g. Extended Tags) regardless.
FAOD, by 'custom field' I mean only a field for which mp3tag does not specifically document a placeholder. e.g. my %fidelity%, as opposed to e.g. %artist%.
I add a field to the mp3tag only where e.g. I need it to appear in a tag panel field or a track list column.
Any field name is permitted in a filter, whether it exists in your files or not. So why would you need custom fields included in a drop down list? Anything that isn't an information field (prefixed with an underscore _) is interpreted as a field name.
The information fields in the filter drop down list are pretty much all covered on this help page:
FWIW, I'd never even think to use the drop down lists to enter a filter. Correct entry of field names is the least of the challenges when entering a filter. One thing I vehemently dislike about Mp3tag's filtering interface is that a bump of the mouse's scroll wheel means that you lose the expression that you were entering, as it scrolls to a saved filter expression in that drop down list.
Does anyone know how to reduce the ever growing picklist to the really needed field names?
How to clean the picklist from all the experimental tag field names?
Have you ever entered TEST3 as the field name in an action? Mp3tag doesn't get those field names from metadata that it reads from files, it gets them from user input. I just tested using a quick Format Value action with a made-up field name. Then I went to the filter drop down lists and it was immediately available in the Extended Fields.
I thought this has been discussed here fairly often. You can pull up a Format Value (Quick) action (or any other action with a Fields drop down list) and click the right-arrow next to the drop down arrow. Select 'Reset fields...' from the menu and confirm. Looks like it removes any custom/experimental field names from all lists.
To selectively prune the list it looks like you may have to do it one field name at a time. Pull up the same quick Format Value action, enter a field name (or select it from the picklist) and then either click the right-arrow and select 'Remove field' from the menu, or use Shift+Del.