Yes, that's the thumbnail browser. It is targeted at artist and album type fields, which have images to show.
I beg to differ!
For one, there's a setting at the bottom of the 'Thumbnail Browser' to 'Don't Display Thumbnails'.
Secondly, many of the exposed default tag field do not come with or warrant using custom thumbnails - Genre, Original Year, Year, Folder Name, Grouping, etc, just to name a few.
And thirdly, this for me (subjectively preferably with the 'Don't Display Thumbnails' option to allow for more metadata to be shown) is one of my central UI elements & filter units which I prefer over all others I have tried out as it does give quick access to most tag types I care about and tend to switch to/fro and does not take up a lot of UI space.
I will test around with the columns browser though now that you pointed out how to customize it (thanks for that muchly!), restricted to a single cell/attribute/filter and see if that does the same (i.e. have a small but quick to switch/navigate central attribute filter window), but also allows me to use the default tags that are inaccessible through ThumbnailBrowser.
Churs.
c.
