When in a trade window, right click any tradeable item to put it in the first available trade slot. Do nothing when all 9 slots are filled. If items are stackable, bring up the prompt to enter the quantity. If it's a single item, place in trade window. Can work for merchant/stores too instead of clicking and dragging to set up shop. A bonus: right click equips to equip (item window) / unequip (equip window) when not in trade window to save double clicks. (Do not use consumables since that's more error prone) This can save a lot time clicking and dragging when transferring gear or storage mules.
