DTX

div {
  padding: 20px;
  text-align: center;
}

var ttl = 0;
var prevS, prevScrollDir;
$(window).on('scroll', function() {
  var s = $(this).scrollTop();
  var scrollDir = s > prevS;
  
  if (scrollDir == prevScrollDir) {
    ttl += (s-prevS);
    if (ttl > 500) {
      $(".m").removeClass('hide');
    }
  } else {
    ttl = 0;
    $(".m").addClass('hide');
  }
  
  prevS = s;
  prevScrollDir = scrollDir;
});

{
  id: 123,
  author_id: 321,
  author: 'Jhon Doe',
  txt: 'A long time ago...'
}

my-app {
    display: flex;
    flex-direction: column;
}

html {
	min-height: 100%;
	height: 100%;
}
body {
  min-height: 100%;
  height: 100%;
  padding: 0;
}
my-app {
  height: 100%;
	margin: 0;
	padding: 0 15px;
	display: -webkit-flex;
	display: flex;
	flex-direction: column;
	flex: auto;
}