Added the multiSelect option to stop event propagation when clicking on the dropdown

app/assets/javascripts/due_date_select.js

@@ -44,6 +44,7 @@ class DueDateSelect {
         this.$selectbox.hide();
         this.$value.css('display', '');
       },
+      multiSelect: true
     });
   }
 

changelogs/unreleased/201427-issue-board-due-date-picker-unable-to-select-custom-year-or-month.yml

+---
+title: Added the multiSelect option to stop event propagation when clicking on the
+  dropdown
+merge_request:
+author: Gwen_
+type: fixed