Unable to take screenshots when inventory is open. Also a wierd little bug with the armor GUI.

So, I was playing survival over on my private server with my friend and I happen to cross by this bug where I can't take screenshots while the Inventory GUI was up. Also, the texture for the leggings slot in the Armor GUI was covered by a black thing. I was only able to take the picture down below through the 'Prt Scr' button.