Comment 14 for bug 1420334

Another interesting case is how to handle tabs being dragged out of Chrome/Chromium. In such a case the window being dragged does not exist for the start of the gesture; it only appears during the drag. Furthermore, the cursor coordinate in the newly created drag window is on the tab itself and not the titlebar.

We have the benefit of knowing that drag gestures in Mir continue to send events to the window where they started until a button is released. So it seems the answer is that this is a matter of the parent window just creating and moving a child. It's not a shell operation but an application operation to move such a window.