XDG toplevel drag

This protocol enhances normal drag and drop with the ability to move a window at the same time. This allows having detachable parts of a window that when dragged out of it become a new window and can be dragged over an existing window to be reattached.

A typical workflow would be when the user starts dragging on top of a detachable part of a window, the client would create a wl_data_source and a xdg_toplevel_drag_v1 object and start the drag as normal via wl_data_device.start_drag. Once the client determines that the detachable window contents should be detached from the originating window, it creates a new xdg_toplevel with these contents and issues a xdg_toplevel_drag_v1.attach request before mapping it. From now on the new window is moved by the compositor during the drag as if the client called xdg_toplevel.move.

Dragging an existing window is similar. The client creates a xdg_toplevel_drag_v1 object and attaches the existing toplevel before starting the drag.

Clients use the existing drag and drop mechanism to detect when a window can be docked or undocked. If the client wants to snap a window into a parent window it should delete or unmap the dragged top-level. If the contents should be detached again it attaches a new toplevel as described above. If a drag operation is cancelled without being dropped, clients should revert to the previous state, deleting any newly created windows as appropriate. When a drag operation ends as indicated by wl_data_source.dnd_drop_performed the dragged toplevel window's final position is determined as if a xdg_toplevel_move operation ended.

Warning! The protocol described in this file is currently in the testing phase. Backward compatible changes may be added together with the corresponding interface version bump. Backward incompatible changes can only be done by creating a new major version of the extension.