[Inventory] Players lose any items held with mouse upon disconnect

I've ran a server for 2 years now, and I've noticed this bug has always been present in the game: if a player selects any item or stack of item in his/her inventory or a chest and is holding it with the mouse when either the server or client disconnects, then the items being held by the mouse will be lost forever, even upon reconnecting. Happens often if a server crashes or the now standard netty.io error.

Please fix this issue within the new inventory handling.
Perhaps do not save inventory data until items are placed into a slot?