Firefox, Xubuntu Linux.
Hit the show text twice, and suddenly, in the bottom corner, shows up 'Javascript:void(0)'. If you mouse over it, it pops up in the opposite bottom corner.
If you are hitting space to page down, it remains in the bottom corner. If you scroll around, it goes away - until the next time.
It's not a bug, as such, this is more an informative post.
edit - update. It specifically disappears when you trigger the "scrolling, settings, and navigation" floater (which I find mildly irritating, but not yet enough to go searching for an off switch)
So it's a synergy between the text click options and the floater.