Hi Graham, I have clients experiencing the same thing with mxCalendar, however I am not. The only difference is that I am using a Mac and they are on Windows, we are all using Firefox 31.0, they have tried other browsers and get the same result. I'd be interested to know if you found a solution.
I would guess that the "top:-200px;" value must be being set by a script that is taking display parameters and creating appropriate positioning values, but ending up wrong because the script is calculating a minus value based on the display info given. So there must be a difference in how the Mac OS and Windows supply the display information, which leads me to think that the positioning script needs to include code to get the OS info and adjust the calculation accordingly, or there is something else wrong with the positioning script. So either way I guess you could call this a bug.
Hopefully Charless will see this and offer a fix at some point
Cheers - Mark