Добавил кнопку прокрутки вверх

src/blocks/to-top/img_to_bg/to-up.svg

+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 17 11" width="17" height="11"><path fill="#fff" d="M.202 1.97L1.956.204A.61.61 0 0 1 2.43 0a.61.61 0 0 1 .47.204L8.5 5.84 14.1.204A.61.61 0 0 1 14.57 0a.61.61 0 0 1 .474.204l1.754 1.766a.618.618 0 0 1 .202.473.62.62 0 0 1-.202.476l-7.826 7.875A.61.61 0 0 1 8.5 11a.61.61 0 0 1-.472-.204L.202 2.92A.618.618 0 0 1 0 2.444c0-.203.067-.34.202-.475z"/></svg>

src/blocks/to-top/to-top.html

+<!--DEV
+
+Нужно убрать пробел между @-ами:
+
+@ @include('blocks/to-top/to-top.html')
+
+-->
+<div class="to-top">content</div>

src/blocks/to-top/to-top.js

+$( document ).ready(function() {
+
+  var t0;
+
+  $('#toTop').hide().on('click', function(event){
+    event.preventDefault();
+    $('body').animate({scrollTop:0}, 300);
+  });
+
+  $(window).on('scroll', function(){
+    clearTimeout(t0);
+    t0 = setTimeout(function () {
+        showScrollTopButton();
+        console.log('scroll');
+      }, 50);
+  });
+
+  showScrollTopButton();
+
+  function showScrollTopButton(){
+    if ( $(document).scrollTop() >= 500 ) {
+      $('#toTop').fadeIn();
+    }
+    else {
+      $('#toTop').fadeOut();
+    }
+  }
+
+});

src/blocks/to-top/to-top.less

+// Для импорта в диспетчер подключений: @import './src/blocks/to-top/to-top.less';
+
+@import '../../less/variables.less';     // только для удобства обращения к переменным
+
+
+.to-top {
+  position: fixed;
+  right: 2rem;
+  bottom: 2rem;
+  z-index: 1;
+  width: 4rem;
+  height: 4rem;
+  border-radius: 50%;
+  background-color: #000;
+  color: transparent;
+  font: 0/0 a;
+  background-image: data-uri('image/svg+xml;charset=UTF-8', './src/blocks/to-top/img_to_bg/to-up.svg');
+  background-repeat: no-repeat;
+  background-position: center;
+  transform: rotate(180deg);
+
+  .js & {
+    display: none;
+  }
+}

src/blocks_library.html

@@ -4666,6 +4666,8 @@
 
 </section>
 
+<a href="#" class="to-top" id="toTop">↑</a>
+
 @@include('blocks/page-footer/page-footer.html')
 @@include('_include/page_bottom.html')
 <script>hljs.initHighlightingOnLoad();</script>

src/less/style.less

@@ -70,3 +70,4 @@
 @import './src/blocks/code/code.less';
 @import './src/blocks/collapse/collapse.less';
 @import './src/blocks/toggler-burger/toggler-burger.less';
+@import './src/blocks/to-top/to-top.less';