What I'm doing:

I control-click to select a couple of genres, I then control-click to
select a couple of artists.  This works as expected, and my playlist then
contains the list of songs from those artists.

However, when I want to add another genre to the list by control-clicking,
the "artist" (and "album") filters get reset to "all".  This gets very
annoying if it's a long list.  Rather than resetting to "all" the list
should just expand to add the artists/albums from the new genre, keeping
the current selection.  If a genre is removed, just unselect/remove the
artists/albums that were in that genre, but don't reset things to "all".

This also applies to selecting/unselecting artists/albums.