1. Drag Operation Modes

Both the drag and the drop protocol are build around X client messages which are send forth and back between the initiator and receiver of the drag and drop operation. Such messages are emitted during the start, cancelation or end of a drag and drop operation, or as the user moves the mouse pointer around the screen and the pointer enters and leaves valid drop sites. This way, the initiator client and the potential receiver client(s) can provide to the user some visual feedback about the current state of the drag operation. Such a visual feedback could be highlighting a drop site so the user notices the existence of a valid drop site. There are two sets of visual feedbacks: the ``drag-over visuals'' of the initiator and the ``drag-under'' visuals of the receiver(s).

The drag operation as well as the handling of the drag-under visuals are different depending on the operation mode of the drag:

• When both the initiator and receiver have agreed to use the dynamic mode they exchange protocol messages dealing (for example) with entering and leaving drop sites during the drag operation. The advantages of this mode are that the X server is not grabbed and can still respond to other event sources, and the receiver can decide what data to accept on-the-fly. On the other side, the dynamic operation mode requires more overhead on behalf of the application and the network.

• The preregister mode is the other possible mode of operation (the appropriate protocol is still undocumented). In this mode, the M*TIF toolkit handles on behalf of the initiator client the complete processing of the drag-under visuals which would ordinarily occour in the receiver client. The receiver is not involved in the process until the drop stage begins. But it has to supply information about drop sites so the initiator can handle the drag-under visuals accordingly. This mode minimises network trafic, but the drop site can't determine whether it wants to accept the drop data until the drop actually occurs. In addition, the server is grabbed during the drag operation.