Fixed firefox tri-layout grid issue and added tablet sticky sidebar

- Fixed issue with original left-sidebar content being placed halfway
down the page.
- Added sticky sidebar to mid-size tablet layout, only for original left
sidebar items.

Fixes #1434.

resources/assets/js/components/page-display.js

@@ -23,8 +23,11 @@ class PageDisplay {
         const sidebarPageNav = document.querySelector('.sidebar-page-nav');
         if (sidebarPageNav) {
             DOM.onChildEvent(sidebarPageNav, 'a', 'click', (event, child) => {
+                event.preventDefault();
                 window.components['tri-layout'][0].showContent();
-                this.goToText(child.getAttribute('href').substr(1));
+                const contentId = child.getAttribute('href').substr(1);
+                this.goToText(contentId);
+                window.history.pushState(null, null, '#' + contentId);
             });
         }
     }