Mouse pointer "parallax" bug in calendar

[srtech]

After around the first half dozen rows of the calendar (around 7 am), the slot after the one clicked by the mouse pointer is selected

For instance (see image below), I clicked on the 11:00 to 11:30 slot using the mouse pointer but the 11:30 to 12:00 time slot was selected.

[atd]

I have been looking closer at this. It gets worse at the rows at the bottom of the calendar. It seems the fullcalendar plugins expects the rows to be one pixel higher at least. So they turn de-synchronized

/cc @crispinh2o