Mir doesn't support drag and drop (DND) windows

[Daniel van Vugt]

Mir doesn't support drag and drop (DND) windows.

In playing with X apps under Xmir, one of the more broken features is drag and drop (_NET_WM_WINDOW_TYPE_DND). Mir doesn't understand it yet, so at best you get a new window appear for the thing getting dragged, but it doesn't move.

Test cases:
  In Xmir run nautilus and drag an icon from it.
  In Xmir run chromium-browser and drag a browser tab from it.

Although X historically implements these things with absolute placement (which Mir doesn't support), we could implement DND movement in Mir shells using some simple heuristic:
   relative_grab_pos = cursor pos in the original app window when the DND window was created
   Now Mir always places the DND window at the cursor pos minus relative_grab_pos.
   Thus the surface being dragged always retains the same position under the cursor as when it was grabbed.

[Daniel van Vugt (vanvugt)]

Oh. The above heuristic only works for Chromium tabs. Not dragging icons from a window like nautilus.

[Daniel van Vugt (vanvugt)]

And maybe Chromium is not an example at all. Under Unity7 it appears Chromium tab dragging is actually dragging a regular window via relative self-movement (so that would be bug 1420334 instead).

[Daniel van Vugt (vanvugt)]

The good news then, it seems, is that we could probably handle DND windows in Mir by simply sticking them on the centre of the cursor. However since Mir prevents clients from knowing they're moving the receiver wouldn't know some DND surface is above it. More DND-specific work would be needed in Mir too.

[Michał Sawicz (saviq)]

Syncing task from Mir.