Why the top value of popup dialog is changed after popupbeforeposition event

问题

I want my popup dialog to show at the top of the screen at each time, so, i add the logic in JS as follows.

$( "#dress_length_size_chart_popup" ).on({
    popupbeforeposition: function () {
        var f = document.getElementById('dress_length_size_chart_popup-popup');
        f.style.top = ** +'px';
    }
});

Unfortunately, it doesn't work. Although, the top is set in the callback function of popupbeforeposition, the value will be changed again, and my settings are overridden. (what happened here??)

If I move the logic to other callback function of popupafteropen. It will work.

However, when I scroll the page, only popupbeforeposition but not popupafteropen will be called. So, the top setting is still lost.

So, the questions are

1. What is happened after popupbeforeposition to make the top value changed?

2. How I can catch the event to make my top setting work?

thanks!

来源：https://stackoverflow.com/questions/34806351/why-the-top-value-of-popup-dialog-is-changed-after-popupbeforeposition-event