World of Warcraft action bars are this way by default, and I imagine other MMOs do as well. Drag a spell onto the bar to be able to use it more easily. Click it

World of Warcraft action bars are this way by default, and I imagine other MMOs do as well. Drag a spell onto the bar to be able to use it more easily. Click it (or, ideally, press an associated hotkey), and it casts the spell or uses the item. Drag the button, and it will make it so that you are again moving the spell around to put it on a different action bar slot (or remove it). Some of them make it so that you have to hold shift when dragging, and that would play nicely with act-on-press, since you already know that the shift-click indicates the start of a drag.