function getOffsetRect(elem) {
    // (1)
    var box = elem.getBoundingClientRect()
    
    // (2)
    var body = document.body
    var docElem = document.documentElement
    
    // (3)
    var scrollTop = window.pageYOffset || docElem.scrollTop || body.scrollTop
    var scrollLeft = window.pageXOffset || docElem.scrollLeft || body.scrollLeft
    
    // (4)
    var clientTop = docElem.clientTop || body.clientTop || 0
    var clientLeft = docElem.clientLeft || body.clientLeft || 0
    
    // (5)
    var top  = box.top +  scrollTop - clientTop
    var left = box.left + scrollLeft - clientLeft
    
    return { top: Math.round(top), left: Math.round(left) }
}