[Playing on mac] I was trying to send some food (Bowl of Simple Stirfry) to my friend through mail, but I had a stack of 11 and only wanted to send him 1. So I alt-dragged to split the stack (the stack was in the mail window and I split to my inventory) and wrote 10 as the new stack size. When I did the item disappeared from the letter I was writing and only the stack of 10 appeared in my inventory. The same thing happened later with a Bolt of Damask that I was trying to send back to the same person.