Why is position().top changing when I scroll the parent?

问题

jQuery position() returns

the current coordinates of the first element in the set of matched elements, relative to the offset parent.

So, scrolling the parent is not supposed to change the position, right?

The result I'm getting in this fiddle is that after scrolling the parent by 100px, the position().top of a child element changes by 100.

position().top before scroll 1880, after scroll 1780

Why?

回答1:

To answer the question in your comments, just add the box's scrollTop to the anchored element's position.

http://jsfiddle.net/5xqEL/17/

var $box = $('#box'),
    $anchored = $('#anchored'),
    $debug = $('#debug');

$debug.text('position().top before scroll ' + ($anchored.position().top + $box.scrollTop()));

$box.animate({
    scrollTop: 100
}).promise().then(function () {
    $debug.text($debug.text() + ', after scroll ' + ($anchored.position().top + $box.scrollTop()));
});

来源：https://stackoverflow.com/questions/20691021/why-is-position-top-changing-when-i-scroll-the-parent