Clicking on the gantt field when gantt header (months / days/ weeks) is scrolled off screen causes an error

[glende...@gmail.com]

When dealing with large gantt charts, if the month / days / weeks header area is scrolled off screen and then clicking on the gantt field, a .js error is thrown.

For the document.emlementFromPoint method, then x,y coordinates must be in the visible area.  So, when the fn-label is out of view, an error is thrown.