I wasn’t sure how I might register a potential bug, so I thought I’d post it here with my possible fix:
Details of Event:
Browser: FireFox
Scheduler View: month
Activity: Dragging events around the month
Description:
I had a problem recently whereby if I scrolled down the page and then starting dragging an event, the event would pop up to the same day of the first week.
After lots of debugging I discovered that the scheduler._mouse_coords function is not taking scroll height into account (although it is for IE).
So, this line:
pos.y-=getAbsoluteTop(this._obj)+this.xy.nav_height+(this._dy_shift||0)+this.xy.scale_height-this._els["dhx_cal_data"][0].scrollTop;was removing too much from pos.y. When pos.y was translated into a date further down the function it had to defer to the earliest week because pos.y wasn’t even on the calendar.
Fix:
My workaround was to change this:
if (!_isIE && (ev.pageX || ev.pageY))
pos={x:ev.pageX, y:ev.pageY};
else pos={
x:ev.clientX + (b.scrollLeft||d.scrollLeft||0) - b.clientLeft,
y:ev.clientY + (b.scrollTop||d.scrollTop||0) - b.clientTop
};to this:
if (!_isIE && (ev.pageX || ev.pageY))
pos={x:ev.pageX, y:ev.pageY + window.pageYOffset};
else pos={
x:ev.clientX + (b.scrollLeft||d.scrollLeft||0) - b.clientLeft,
y:ev.clientY + (b.scrollTop||d.scrollTop||0) - b.clientTop
};
Note: It’s entirely possible that the X coord would need modifying as well, although people are less likely to lay their scheduler out on a page where they’d need to scroll right to get to events.
Just thought someone at dhtmlx might want to know 
PS: this is my first forum post, and I’d like to say that we LOVE dhtmlxScheduler!