For example, you have a stack of stonebrick walls in your inventory and one on your hotbar. You should be able to oepn your inventory and double click on the walls or shift+click and it will transfer to the stack of walls in the hotbar. This should happen consistantly . . . with every item.
Also, quite often when dropping a bunch of things (i often have this with the cheap fiber armor that thralls wear straight out of the wheel) my inventory suddenly jumps to the top item or another random item in my inventory instead of the next item to be dropped. I then end up dropping items that i need and i have to scroll down to select the armor again that i want to drop.