The description is at en.wp,
I hope my TL;DR of this visual glitch is ok: if you hover on a template with your mouse, hence getting the blue highlight to show, but then move the template down in the page (i.e. by adding newlines, with your keyboard), the highlight will stay in the original position, until you move the mouse again.
I honestly have no idea whether there's really a bug here, but still.
Here's a video!