The javascript module expect the itemcode to be the key of the item in the configuration array. This fixes the partial to use the code instead of the 0-indexed value.

Should close #1309

modules/backend/layouts/_sidenav.htm

@@ -21,7 +21,7 @@
                                 </a>
                                 <span
                                     class="counter <?= $item->counter === null ? 'empty' : null ?>"
-                                    data-menu-id="<?= e($context->mainMenuCode.'/'.$sideItemCode) ?>"
+                                    data-menu-id="<?= e($context->mainMenuCode.'/'.$item->code) ?>"
                                     <?php if ($item->counterLabel): ?>title="<?= e(trans($item->counterLabel)) ?>"<?php endif ?>
                                     >
                                     <?= e($item->counter) ?>