This was driving me absolutely nuts. Thanks for letting us in on the secret, Zoltan. This cuts down on the frustration immensely!
It was driving me nuts also when I was first playing. I actually discovered this worked by accident...sort of. I remember watching a review of the game on youtube before I bought it, to see if it was my type of game....and I recalled seeing the reviewer moving whole stacks of items at once, but I couldn't seem to figure out how after I started playing. I was double clicking, ect., and then just by trial and error, I tried the shift key, and bingo, lol. Glad you stumbled across this Jinx. The game's not perfect (especially the combat), but it is a lot of fun never the less!