gitlab-org--gitlab-foss/app/assets/javascripts/layout_nav.js

import ContextualSidebar from './contextual_sidebar';
import initFlyOutNav from './fly_out_nav';

function hideEndFade($scrollingTabs) {
  $scrollingTabs.each(function scrollTabsLoop() {
    const $this = $(this);
    $this.siblings('.fade-right').toggleClass('scrolling', Math.round($this.width()) < $this.prop('scrollWidth'));
  });
}

export default function initLayoutNav() {
  const contextualSidebar = new ContextualSidebar();
  contextualSidebar.bindEvents();

  initFlyOutNav();

  $(document).on('init.scrolling-tabs', () => {
    const $scrollingTabs = $('.scrolling-tabs').not('.is-initialized');
    $scrollingTabs.addClass('is-initialized');

    $(window).on('resize.nav', () => {
      hideEndFade($scrollingTabs);
    }).trigger('resize.nav');

    $scrollingTabs.on('scroll', function tabsScrollEvent() {
      const $this = $(this);
      const currentPosition = $this.scrollLeft();
      const maxPosition = $this.prop('scrollWidth') - $this.outerWidth();

      $this.siblings('.fade-left').toggleClass('scrolling', currentPosition > 0);
      $this.siblings('.fade-right').toggleClass('scrolling', currentPosition < maxPosition - 1);
    });

    $scrollingTabs.each(function scrollTabsEachLoop() {
      const $this = $(this);
      const scrollingTabWidth = $this.width();
      const $active = $this.find('.active');
      const activeWidth = $active.width();

      if ($active.length) {
        const offset = $active.offset().left + activeWidth;

        if (offset > scrollingTabWidth - 30) {
          const scrollLeft = (offset - (scrollingTabWidth / 2)) - (activeWidth / 2);

          $this.scrollLeft(scrollLeft);
        }
      }
    });
  }).trigger('init.scrolling-tabs');
}