javascript - Stop fixed element scrolling at certain point

Question

I have fixed sidebar which should scroll along with main content and stop at certain point when I scroll down. And vise versa when I scroll up.

I wrote script which determines window height, scrollY position, position where sidebar should 'stop'. I stop sidebar by adding css 'bottom' property. But I have 2 problems with this approach:

1. When sidebar is close to 'pagination' where it should stop, it suddenly jumps down. When I scroll up it suddenly jumps up.
2. When I scroll page, sidebar moves all the time

Here's my code. HTML:

<div class="container">
  <aside></aside>
  <div class="content">
    <div class="pagination"></div>
  </div>
  <footer></footer>
</div>

CSS:

aside {
  display: flex;
  position: fixed;
  background-color: #fff;
  border-radius: 4px;
  transition: 0s;
  transition: margin .2s, bottom .05s;
  background: orange;
  height: 350px;
  width: 200px;
}

.content {
  display: flex;
  align-items: flex-end;
  height: 1000px;
  width: 100%;
  background: green;
}

.pagination {
  height: 100px;
  width: 100%;
  background: blue;
}

footer {  
  height: 500px;
  width: 100%;
  background: red;
}

JS:

let board = $('.pagination')[0].offsetTop;
let filterPanel = $('aside');
    if (board <= window.innerHeight) {
        filterPanel.css('position', 'static');
        filterPanel.css('padding-right', '0');
    }

$(document).on('scroll', function () {
    let filterPanelBottom = filterPanel.offset().top + filterPanel.outerHeight(true);
    let bottomDiff = board - filterPanelBottom;

    if(filterPanel.css('position') != 'static') {

        if (window.scrollY + window.innerHeight - (bottomDiff*2.6) >= board)
            filterPanel.css('bottom', window.scrollY + window.innerHeight - board);
        else
            filterPanel.css('bottom', '');
    }
});

Here's live demo on codepen

Side bar is marked with orange background and block where it should stop is marked with blue. Than you for your help in advance.

Answer 1

I solved my problem with solution described here

var windw = this;
let board = $('.pagination').offset().top;
let asideHeight = $('aside').outerHeight(true);
let coords = board - asideHeight;
console.log(coords)
$.fn.followTo = function ( pos ) {
    var $this = this,
        $window = $(windw);
        
    $window.scroll(function(e){
      
        if ($window.scrollTop() > pos) {
            $this.css({
                position: 'absolute',
                top: pos
            });
        } else {
            $this.css({
                position: 'fixed',
                top: 0
            });
        }
    });
};

$('aside').followTo(coords);

And calculated coordinates as endpoint offset top - sidebar height. You can see solution in my codepen