closes #3767 closes #3774

public/src/client/category.js

@@ -104,7 +104,9 @@ define('forum/category', [
 			var bookmarkIndex = localStorage.getItem('category:' + cid + ':bookmark');
 			var clickedIndex = localStorage.getItem('category:' + cid + ':bookmark:clicked');
 
-			if (!bookmarkIndex) {
+			bookmarkIndex = Math.max(0, parseInt(bookmarkIndex, 10) || 0);
+			clickedIndex = Math.max(0, parseInt(clickedIndex, 10) || 0);
+			if (!parseInt(bookmarkIndex, 10)) {
 				return;
 			}
 
@@ -123,13 +125,9 @@ define('forum/category', [
 					return;
 				}
 
-				if (bookmarkIndex < 0) {
-					bookmarkIndex = 0;
-				}
-
 				$('[component="category"]').empty();
 
-				loadTopicsAfter(bookmarkIndex - 1, 1, function() {
+				loadTopicsAfter(Math.max(0, bookmarkIndex - 1), 1, function() {
 					Category.scrollToTopic(bookmarkIndex, clickedIndex, 0);
 				});
 			}

public/src/modules/navigator.js

@@ -95,11 +95,14 @@ define('navigator', ['forum/pagination', 'components'], function(pagination, com
 	navigator.update = function() {
 		toggle(!!count);
 
-		var middleOfViewport = $(window).scrollTop() + $(window).height() / 2;
+		var els = $(navigator.selector);
+		if (els.length) {
+			index = parseInt(els.first().attr('data-index'), 10) + 1;
+		}
 
-		index = parseInt($(navigator.selector).first().attr('data-index'), 10) + 1;
+		var middleOfViewport = $(window).scrollTop() + $(window).height() / 2;
 		var previousDistance = Number.MAX_VALUE;
-		$(navigator.selector).each(function() {
+		els.each(function() {
 			var distanceToMiddle = Math.abs(middleOfViewport - $(this).offset().top);
 
 			if (distanceToMiddle > previousDistance) {