How about, instead of opening up a backpack, possibly scrolling down until you find free space and then transferring items to that free space, you just left-click an item, then right-click on the backpack - the item being put into the first available slot? Makes rearranging one's inventory less time-consuming a task..

Not sure if this is the right forum to post this in, but it has to do with gameplay, and isn't a bug. Can always repost elsewhere if necessary.