I don't know how I do it but sometimes a track gets collapsed completely (to about 3 pixels of height).
I can easily drag it back to a normal size but I can't find how this happened and I would like to know, as I would find it useful to do it on command do de-clutter the timeline at times.

Is it even an intended feature, and where's the button for it?

Minimum track height is 19. How do you manage to get a 3 height track?

I don't know. Next time it happens I'll post a screenshot and a feature request.