Carousel arrow buttons: hide focus states when not hovering
As pointed out in !621 (comment 1039476), keeping the focus state visible on hover means it will be visible the entire time from when you use an arrow to leaving the view, because in GTK3 there's no timeout for focus. This means they're very likely to show up on e.g. screenshots (see the ones in this comment for example).
The 10% thing is not a solution here, because it also looks like a bug (it looks like the button is "stuck" somehow).
Seeing as this is a highly visible glitch which we can't fix in GTK3 I really think we should go back to hiding it on non-hover.