fixes #3471

Closes #3471

Merge request !2366

resources/assets/javascripts/bootstrap/forms.js

@@ -257,24 +257,27 @@ STUDIP.ready(function () {
     $('select.nested-select:not(:has(optgroup)):hidden:not(.select2-awaiting)').each(function() {
         var observer = new window.MutationObserver(function(mutations) {
             mutations.forEach(function(mutation) {
-                if ($('select.select2-awaiting', mutation.target).length > 0) {
-                    $('select.select2-awaiting', mutation.target)
-                        .removeClass('select2-awaiting')
-                        .each(function() {
-                            createSelect2(this);
-                        });
+                let targets = Array.from(mutation.target.querySelectorAll('select.select2-awaiting'));
+                if (mutation.target.matches('select.select2-awaiting')) {
+                    targets.push(mutation.target);
+                }
+                targets = $(targets).filter(':visible');
+
+                if (targets.length > 0) {
+                    targets.removeClass('select2-awaiting').each(function() {
+                        createSelect2(this);
+                    });
                     observer.disconnect();
-                    observer = null;
                 }
             });
         });
         observer.observe($(this).closest(':visible')[0], {
             attributeOldValue: true,
             attributes: true,
-            attributeFilter: ['style', 'class'],
+            attributeFilter: ['style', 'class', 'hidden'],
             characterData: false,
             childList: true,
-            subtree: false
+            subtree: true
         });
 
         $(this).addClass('select2-awaiting');