When you look at a backpack, it has a little icon at the bottom middle, its a minimize button.

When you click that button, you get this...

You can move it anywhere on your screen and drop items on it, and it goes in your pack... but.. when the items go in your bag, they drop in the upper left hand corner, and DO NOT stack, i dropped about 20 piles of gold on my minimized bag icon, and i had 20 piles of gold in my bag, in the upper left hand corner.
not sure if its a bug, but sure seems like it...

When you click that button, you get this...

You can move it anywhere on your screen and drop items on it, and it goes in your pack... but.. when the items go in your bag, they drop in the upper left hand corner, and DO NOT stack, i dropped about 20 piles of gold on my minimized bag icon, and i had 20 piles of gold in my bag, in the upper left hand corner.
not sure if its a bug, but sure seems like it...