Cursor position reported by event.pageX and event.pageY is out by a couple of pixels, due to clientX and clientY starting from 2,2 in standards mode (0,0 in quirks mode).

The solution is to modify the fix method of jQuery.event such that...

// Calculate pageX/Y if missing and clientX/Y available
if ( event.pageX == null && event.clientX != null ) {
  var e = document.documentElement, b = document.body;
  event.pageX = event.clientX + (e && e.scrollLeft || b.scrollLeft || 0);
  event.pageY = event.clientY + (e && e.scrollTop || b.scrollTop || 0);
}

becomes...

// Calculate pageX/Y if missing and clientX/Y available
if ( event.pageX == null && event.clientX != null ) {
  var e = document.documentElement||{}, b = document.body||{};
  event.pageX = event.clientX + (e.scrollLeft || b.scrollLeft || 0) - (e.clientLeft || 0);
  event.pageY = event.clientY + (e.scrollTop || b.scrollTop || 0) - (e.clientTop || 0);
}

NB. Solution/explanation based on

http://ajaxian.com/archives/javascript-tip-cross-browser-cursor-positioning