Item stuck on cursor, can't interact with the game using mouse

Inventory full (as shown in image), picked up an item which did not fit in it, while the inventory window was open. Now I can't interact with the game (move, click on anything) except for swapping the item with another item in my inventory (namely the maul, which has enough space to fit the item I picked up).

I expect it to be possible to throw an item back to the ground, if it does not fit in inventory.

Workaround for this is to use the /destroy chat command, which worked for in my case. However, this requires prior knowledge of the command. If the person having this issue does not know of this command, they're practically locked out of the game and have to Alt+F4 out.