I just noticed this by accident. A little white line can be seen under the exit button on hovering.
On some resolutions it can't be seen, on some its obscure. Can be seen clearly on:

800x600, 1152x864, 1280x1024, 1024x600, 1280x720, 1440x900, 1600x900, 1600x1024, 1680x1050, 1768x992




osu! version: 20150204cuttingedge

i see

that appears on the edit button as well ^ ^

It's in the stable build too. I can confirm that.

Wait this post is 9 days old what

How'd you even see this?
Such a minor detail
You have some real good eyes

Moved to low priority.
Not something super important afterall.