For people to more easily customize their toolbar they should be able to just drag toolbar buttons to the position they want and release to rearrange the them. The indication that buttons are being rearranged can be the same spacing behavior we have when re-arranging buttons in customize mode. This would not influence any existing behavior but only add a new one on click-drag. To not influence regular click this behavior might be triggered only after a certain amount of dragging. I think 6-8px should be enough.