just in case anyone has any replies or additions I have subscribed to this thread.

I Fixed the bug on my way, tested on IE7, IE6(Multiple IE), FF, and SAFARI, with those lines :

if (document.body.scrollLeft){br.x += document.body.scrollLeft;}
br.x += window.scrollX;
if (document.body.scrollTop){br.y += document.body.scrollTop;}
br.y += window.scrollY;

and it works.

Try this - it uses feature detection instead of useragent detection:

Place these two functions at the top of the file - or in a utility.js or whereever they'll be seen by the Calendar object:

getViewportScrollX = function() {
var scrollX = 0;
if( document.documentElement && document.documentElement.scrollLeft ) { //IE standards compliant and W3C
scrollX = document.documentElement.scrollLeft;
}
else if( document.body && document.body.scrollLeft ) { // IE 6 not standards compliant
scrollX = document.body.scrollLeft;
}
else if( window.pageXOffset ) { // older browsers
scrollX = window.pageXOffset;
}
else if( window.scrollX ) { // Gecko and KHTML/Webkit browsers I think...
scrollX = window.scrollX;
}
return scrollX;
};

getViewportScrollY = function() {
var scrollY = 0;
if( document.documentElement && document.documentElement.scrollTop ) {
scrollY = document.documentElement.scrollTop;
}
else if( document.body && document.body.scrollTop ) {
scrollY = document.body.scrollTop;
}
else if( window.pageYOffset ) {
scrollY = window.pageYOffset;
}
else if( window.scrollY ) {
scrollY = window.scrollY;
}
return scrollY;
};

Replace this section starting at line 1392:

if (Calendar.is_ie) { //Ugh!
br.y += document.body.scrollTop;
br.x += document.body.scrollLeft;
} else {
br.y += window.scrollY;
br.x += window.scrollX;
}

with calls to the above functions:

br.x += getViewPortScrollX();
br.y += getViewPortScrollY();

See here for more than you wanted to know about viewports: