show elem by scroll

scroll elem

let $win = $(window);
let $marker = $('#marker');
let $wrap = $('.wrap');


$win.scroll(function () {
  
  if ($win.scrollTop() + $win.height() * 0.8 >= $marker.offset().top && $win.scrollTop() <= ($marker.offset().top + $marker.height() * 0.8)) {
    $wrap.addClass('full'); 
  } else {
    $wrap.removeClass('full'); 
  }
});


<div class="wrap">
  <div class="sec"></div>
  <div class="sec"></div>
  <div class="sec"></div>
  <div class="sec" id="marker"></div>
  <div class="sec"></div>
  <div class="sec"></div>
</div>