The getpos() function doesn't work very well with multiple screens.
It doesn't return correct tags when using it on other screens than the first.
Here is a corrected version of this function.
In the third commit, I changed the default behaviour to add a tag to every screen if the screen attribute is not defined for the tag.
Return correct tag using getpos with multiple monitors
Rewrite of getpos to correct bugs with multiple screens
Add tag to every screen if tag.screen is not defined on init
d'oh. Sorry, this got buried in my 'TODO' list.