I'm not exactly sure how to describe this.

It was working perfectly fine before now, but now it does what happens in the picture. It happens in the menu and in the game, and causes a small, but noticable, FPS drop.

The strange overlay is not always a perfect rectangle, and not always black. It seems to be sky blue in-game, whilst black in the menu.

EDIT : Seems to happen in Minecraft Forge 1.7.10. I will re-install the launcher.

EDIT2 : Restarting seems to fix it.