Apologies I thought I submitted a screenshot with the post. The value isn't inverted, it's just a little over 2x what I would expect. Moving it from the top of my monitor moves it about half the # of pixels I would expect ~600 vs my monitor height 1280.
Viewing post in mouse_y not what I'm expecting
Very interesting! Its hard for me to be able to tell whats going on from that picture, but maybe it is due to the camera size / Render Quality?
You can change Render Quality in the FAUXTON_INIT() script wherever you are calling it. :P
If that doesn't work, if you could send a video i'd be more than happy to give it a look and see if I can deduce the issue! :)