Благодаря рекомендации MikaelLennholm , проблема решена!
Казалось, что это осложнение document.documentElement.scrollTop
. Заменяя его $(window).scrollTop()
, скрипт теперь работает плавно перекрестно.
Вот фрагмент рабочего скрипта в действии:
$(document).ready(function() {
var $window = $(window);
var div2 = $('#toggle-element');
var div1 = $('#container2');
$window.on('scroll', function() {
var scrollTop = $(window).scrollTop();
var viewport_height = $window.height();
var scrollTop_bottom = scrollTop + viewport_height;
var window_top_to_div2 = ($window.height() - div2.height()) / 2;
var div1_top = div1.offset().top;
var div1_height = div1.height();
var div1_bottom = div1_top + div1_height;
if (scrollTop >= (div1_top - window_top_to_div2) && (scrollTop + window.innerHeight) <= (div1_bottom + window_top_to_div2)) {
div2.addClass('show');
} else {
div2.removeClass('show');
}
});
});
#toggle-element {
height: 50px;
text-align: center;
margin: auto;
top: 0;
bottom: 0;
right: 50%;
left: 50%;
visibility: hidden;
opacity: 0;
-webkit-transition: opacity 500ms, visibility 500ms;
-o-transition: opacity 500ms, visibility 500ms;
transition: opacity 500ms, visibility 500ms;
z-index: 1;
position: fixed;
max-width: 1000px;
width: 100%;
-webkit-transform: translateX(-50%);
-ms-transform: translateX(-50%);
transform: translateX(-50%);
}
#toggle-element.show {
visibility: visible;
opacity: 1;
}
#toggle-element .wrap {
position: relative;
height: 50px;
width: 80%;
margin: 0 auto;
}
#toggle-element .navbtns {
display: table-cell;
width: 50px;
height: 50px;
position: absolute;
top: 0;
background: transparent;
}
#toggle-element .navbtns svg {
fill: blue;
opacity: .8;
overflow: visible;
will-change: opacity;
-webkit-transition: all 0.5s ease;
-o-transition: all 0.5s ease;
transition: all 0.5s ease;
}
#toggle-element .navbtns svg:hover {
opacity: 1;
}
#toggle-element .prev {
right: 0;
margin-right: -25px;
}
#toggle-element .next {
left: 0;
margin-left: -25px;
}
#container1,
#container3 {
width: 60%;
height: 1000px;
background: yellow;
margin: 0 auto;
display: block;
text-align: center;
}
#container2 {
width: 60%;
height: 2000px;
margin: 0 auto;
position: relative;
background: blue;
display: block;
text-align: center;
color: #fff;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="toggle-element">
<div class="wrap">
<a href="#" class="navbtns prev" title="Go to Next Page">
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 50 50"><path d="M25,50A25,25,0,1,0,0,25,25,25,0,0,0,25,50ZM25,2A23,23,0,1,1,2,25,23,23,0,0,1,25,2Zm-3,9V39L32,25Z" style="fill-rule:evenodd"/></svg>
</a>
<a href="#" class="navbtns next" title="Go to Previous Page">
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 50 50"><path d="M25,50A25,25,0,1,1,50,25,25,25,0,0,1,25,50ZM25,2A23,23,0,1,0,48,25,23,23,0,0,0,25,2Zm3,9V39L18,25Z" style="fill-rule:evenodd"/></svg>
</a>
</div>
</div>
<div id="container1">
Scroll down to <b>#container2</b>
</div>
<div id="container2">
This is <b>#container2</b>
</div>
<div id="container3">
Scroll up to <b>#container2</b>
</div>