[dwm] Catching a button press outside a window

How is it possible to catch a buttonpress event outside any window, without
messing up everything?

Until now I've tried to apply grabbuttons to the root window, with
focused=True. Doing so, the root window catches the button press, but it
catches too much, i.e. every button press. So every other window becomes deaf.

I've tried also to create a ghost window with CWOverrideRedirect, like barwin
does, which has the same size of the root window. It works.
However, since I don't update it, old pixels remains when a window is moved.

Is there a simple solution, or do I have to update the ghost win every time?

Thanks ;)

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