These docs are for an old release.
Info on upgrading to v6
fixedMirrorParent
The parent element that fixed-positioned event mirror elements are attached to while dragging.
By default, dragged mirror elements are attached to the main calendar <div>.
This setting is useful for when you have complex CSS transforms on the main calendar <div> that can distort fixed-positioned elements within. You should then set the element to an outer to avoid this distortion. Example:
var calendar = new Calendar(calendarEl, {
fixedMirrorParent: document.body
});