Background: When you open an inventory, the client does not send a packet to the server. However, when you close the inventory, a packet is sent. This behavior prevents you from using the crafting grid as a storage space.
KeepThatInventoryOpen is a client-side mod that prevents the client from sending the inventory close packet. This allows you to store items in the crafting grid and the mouse slot. But be aware that these items will get thrown out of the inventory if you rejoin the world/server.
Use at your own risk. Some servers may view this as an unfair advantage.