When attaching a calendar to an input placed inside a div whose position is set to "fixed", the popup calendar doesn't show up in the right position.

The problem is that the calendar use the input's offset (relative to the viewport's top left corner) to compute its position while it should use an offset relative to the document's top left corner.

Workaround:

check whether a parent of the target input field has its position set to 'fixed'. If it is the case, then set the popup position to 'fixed' instead of 'absolute'

Quick fix for ui.calendar:

in the showFor method, line 313, replace

if (!popUpCal._pos) { // position below input

	popUpCal._pos = popUpCal._findPos(input);
	popUpCal._pos[1] += input.offsetHeight;
}
inst._calendarDiv.css('position', (popUpCal._inDialog && $.blockUI ? 'static' : 'absolute')).
	css('left', popUpCal._pos[0] + 'px').css('top', popUpCal._pos[1] + 'px');

By

if (!popUpCal._pos) { // position below input
	popUpCal._pos = popUpCal._findPos(input);
	popUpCal._pos[1] += input.offsetHeight;
	
	// Check whether the input is not inside a position:fixed div
	popUpCal._fixed  = false
	$(input).parents().each(function() {popUpCal._fixed  |= $(this).css('position') == 'fixed'});
}
inst._calendarDiv.css('position', (popUpCal._inDialog && $.blockUI ? 'static' : (popUpCal._fixed ? 'fixed' : 'absolute'))).
	css('left', popUpCal._pos[0] + 'px').css('top', popUpCal._pos[1] + 'px');