JS-到达元素不准确

我想在用户到达某个元素时添加一个CSS类。

var pag_offset = $('.pagination_extra').offset();
$('.pagination-wrapper').addClass('pagination-wrapper-fixed');
$(window).scroll(function() {
   if($(window).scrollTop() + $(window).height() < $(window).height() + pag_offset.top - 100) {
   		$('.pagination-wrapper').addClass('pagination-wrapper-fixed');
   } else {
   		$('.pagination-wrapper').removeClass('pagination-wrapper-fixed');
   }
});
.pagination_extra {
  display: inline-block;
  width: 100%;
}
.pagination-wrapper a {
  text-decoration: none;
  cursor: pointer;
}
.pagination-wrapper {
  text-align: center;
  margin:auto;
  width: 50%;
}
.pagination-wrapper-fixed {
  position: fixed;
  bottom: 0;
  z-index: 7;
  width: 100%;
  margin-bottom: 8px;
}
.pagination {
  display: inline-block;
  height: 70px;
  margin-top: 10px;
  padding: 0 25px;
  border-radius: 35px;
  background-color: #eee;
}
@media only screen and (max-width: 1199px) {
  .pagination {
    height: 50px;
    padding: 0 10px;
    border-radius: 25px;
  }
}

.page-numbers {
  display: block;
  padding: 0 25px;
  float: left;
  transition: 300ms ease;
  color: #595959;
  font-size: 20px;
  line-height: 70px;
  font-family: nazanin;
}
.page-numbers:hover, .page-numbers.current {
  background-color: #86c023;
  color: #fff;
}
.page-numbers.prev:hover, .page-numbers.next:hover {
  background-color: transparent;
  color: #86c023;
}
@media only screen and (max-width: 1199px) {
  .page-numbers {
    padding: 0 15px;
    font-size: 16px;
    line-height: 50px;
  }
}
@media only screen and (min-width: 120px) and (max-width: 1024px) {
  .page-numbers {
    padding: 0 14px;
    display: none;
  }
  .page-numbers:nth-of-type(2) {
    position: relative;
    padding-right: 50px;
  }
  .page-numbers:nth-of-type(2)::after {
    content: "...";
    position: absolute;
    font-size: 25px;
    top: 0;
    left: 45px;
  }
  .page-numbers:nth-child(-n + 3), .page-numbers:nth-last-child(-n + 3) {
    display: block;
  }
  .page-numbers:nth-last-child(-n + 4) {
    padding-right: 14px;
  }
  .page-numbers:nth-last-child(-n + 4)::after {
    content: none;
  }
}
<h1>foo<br></h1><h1>foo<br></h1><h1>foo<br></h1><h1>foo<br></h1><h1>foo<br></h1><h1>foo<br></h1><h1>foo<br></h1><h1>foo<br></h1><h1>foo<br></h1><h1>foo<br></h1><h1>foo<br></h1><h1>foo<br></h1>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="pagination_extra"></div>
<div class="pagination-wrapper">
  <div class="pagination">
    <a class="prev page-numbers">&larr;</a>
    <a class="page-numbers">1</a>
    <a class="page-numbers">2</a>
    <a class="page-numbers">3</a>
    <a class="page-numbers">4</a>
    <a class="next page-numbers">&rarr;</a>
  </div>
</div>
<h1>foo<br></h1><h1>foo<br></h1><h1>foo<br></h1><h1>foo<br></h1><h1>foo<br></h1><h1>foo<br></h1>

When user reaches the .pagination_extra element it should add a CSS class.

问题是,当尚未到达将删除该类的元素时,它是不准确的。

So I tried to add or subtract the offset value like pag_offset.top - 100 but it will change on different screen size.

How can I make it accurate that when user exactly reaches the .pagination_extra element it changes the class?