Weird square around my mouse cursor

There is a weird square around my mouse cursor. This bug did not happen in past versions.
You see, in the pic, the "Singleplayer" button, there's a small gray square on it. My mouse cursor is there. But when I screenshot the mouse cursor disappears.
When I use the screenshot button (F2), the gray square disappears.
EDIT: This bug did not happen to 1.7.9 before. After I played on 1.7.10, it suddenly started to happen in 1.7.9.