Customize - Drop Effects
A drag source can advertise a drop effect through DropEffect, which sets the
native dataTransfer.effectAllowed. Drag the two boxes below and watch the
cursor: the copy box shows a “+” badge, the move box does not.
Copy me (effect = copy)
Move me (effect = move)
Drop here
The browser only offers the effects a source allows. With
DropEffect="copyMove"
the user can switch to copying by holding Ctrl (⌥ on macOS) during the drag;
the boxes above allow a single effect each, so modifier keys do not change them.