TL;DRAllow right click, or some other modifier to always set the quickbar slot, regardless of its current contents.
What ?Currently, if the player has an item held in the cursor and clicks on an empty quickbar slot, the quickbar is set to the item. But if the player clicks on a quickbar slot that already has an item, the cursor is changed to that slot. If the player right clicks the slot, same thing. Shift-clicks, nothing happens. Middle clicks, it clears the slot, but still doesn't set it requiring a second click.
It would be good if their was an action that is currently redundant or does nothing, overrode the quickbar slot with whatever the current cursor item is. For example right-click.