https://feminism-boell.org/themes/theodor/js/back-to-top.js?v=1.2.45

1(function (Drupal, once) {
2  Drupal.behaviors.back_to_top = {
3    attach: function (context, settings) {
4
5      const topButton = document.querySelector('[data-to-top-button]');
6
7      if (!topButton) {
8        return;
9      }
10      topButton.parentNode.classList.add('hidden');
11
12      let lastScrollY = 0;
13
14      /**
15       * Only show the button, if scrolled down and then scrolled up a bit
16       */
17      const updateVisibility = () => {
18        let scrollY = Math.max(
19          window.scrollY,
20          document.documentElement.scrollTop,
21          0
22        );
23
24        /**
25         * scroll-snap content has its own scroll container, which has to be
26         * queried differentl from the window object
27         */
28        const scrollSnap = document.getElementById('scrollytelling-snapping--signifier');
29        if (scrollSnap) {
30          scrollY = scrollSnap.scrollTop;
31        }
32
33        const isVisible = scrollY >= window.innerHeight && scrollY < lastScrollY; // more than viewport height and scrolling up at the moment
34        lastScrollY = scrollY;
35
36        // make sure the scroll-eventlistener doesn't slow down the site
37        window.requestAnimationFrame(() => {
38          if (isVisible) {
39            topButton.parentNode.classList.remove('hidden');
40          } else {
41            topButton.parentNode.classList.add('hidden');
42          }
43        });
44
45      };
46
47      /**
48       * We can use the window object in general, unless we're using scroll-snapping.
49       * In this case we have to specifically listen on the scroll-snapp container.
50       */
51      const scrollContent = document.getElementById('scrollytelling-snapping--signifier') || window;
52      scrollContent.addEventListener('scroll', updateVisibility);
53    },
54  };
55
56})(Drupal, once);