Alright this bug is pretty simple and isn't a massive issue but i have had this before when i have been modding the main menu and changed a few GL11 methods.

So basically if you go onto multiplayer and hover over the 'ping' server area and press escape (takes you back to main menu) the main-menu will go darker and have no panorama.

I believe its some sort of GL issue doing this.

I have attached an image of what the main-menu looks like.