Behavior now: when you click the column headers in songlist or songtree, the list get sorted by that field, alternately ascending and descending.

Suggestion: When you click a column header of a column that is sorted descending, you return to the default sort order. Say you have chosen "Sort by -> path". When you click for example the "Title" header repeatedly, the sorting order goes title asc -> title desc -> path -> title asc. etc. I know at least Spotify has this behavior.

I'd like this because have have my default sort order which I like, but sometimes I use sorting to find a song/album/artist quickly. It would then be nice to return to the default sort order without having to right click and choose from a menu.

Interesting idea, I'm not completely sure how intuitive/consistent it is.
One thing I'm wondering about is what happen if the "default sort order" is just (for example) "Title" or "Title (descending)" and you click on the title column, I guess in that case it would just cycle between the 2. That makes it slightly unpredictable when you click on a column that is sorted in descending order.
I think I'll implement it, but I want to think about it a bit first.
Opinions welcome.

You get a small "^" or "v" to the right in the column header button if you click it to sort. However, if you choose a sort order from the context menu, you don't get that (even if you choose a sort order that corresponds to a column, for example "Title"). So that becomes the indicator. It may seem small, but it works for me. And it is consistent: if you don't see a "^" or "v", you know you're back at default sort order.

You can check out the manual search part of the albuminfo plugin in the shimmer git, I've implemented it there.

Even though there isn't any indicator in the listview-headers (and I don't think it makes much sense putting any there) for the more complex sort-modes, I'm in favor of this triple-clicking of the headers.
It might not be intuitive in the first place, but I think it's one of those small things users will stumble across eventually and appreciate.