Please describe the problem:
When trying to use drag-and-drop to a hidden window, the only way to get to it
is by dragging to the window's entry in the task list, waiting for it to
activate (and brought to the current desktop) and then dropping on it. 

When the target window is part of a window group in the tasklist, there is no
way to access it as dragging to the window group button doesn't do anything.

Steps to reproduce:
1. Make sure some windows are in a window group button - for example by opening
a lot of Firefox windows.
2. grab something that can be dragged - like a text selection or a url - from
any window
3. drag to a normal button on the tasklist and wait w/o releasing- the relevant
window activates.
4. drag to a window group button


Actual results:
Nothing happens

Expected results:
I would expect that the window list for the window group button would open -
like when just clicking it normally does.

Does this happen every time?
It happens everytime

Other information:
The only workaround for this is to make sure that the target window is visible
before starting the drag and drop. This usually requires a lot of window
rearranging though it can be minimized by making sure the target window is
visible on a different desktop and then dragging to the pager. If there is no
alternative to the drag-and-drop action (like copy and paste), then this is a
major hurdle.