Use platform.isSmallScreen() to get correct $scroller element

src/js/theme/navigation.js

@@ -12,7 +12,7 @@ var usePushState = (typeof history.pushState !== 'undefined');
     Get current scroller element
 */
 function getScroller() {
-    if (platform.isMobile()) {
+    if (platform.isSmallScreen()) {
         return $('.book-body');
     } else {
         return $('.body-inner');

src/js/theme/platform.js

+var $ = require('jquery');
+
 module.exports = {
     isMobile: function() {
         return ($(document).width() <= 600);
+    },
+    // Breakpoint for navigation links position
+    isSmallScreen: function() {
+        return ($(document).width() <= 1240);
     }
 };