Atlantis allows you to perform so called Drag & Drop operations (moving text to another location, duplicating text).
To move a text fragment with the mouse,
If you wish to duplicate the selected text (to move not the original text to another location but its copy), hold the Ctrl key while dragging.
Note. The Drag & Drop operations are allowed if the Enable Drag & Drop Editing option (the Tools | Options... command, Edit tab) is on.