Resets issue boards assignee dropdown

Closes #26514

app/assets/javascripts/boards/components/board_sidebar.js.es6

@@ -30,6 +30,10 @@
       detail: {
         handler () {
           this.issue = this.detail.issue;
+
+          if (this.showSidebar) {
+            $('.js-user-search', this.$el).data('glDropdown').clearMenu();
+          }
         },
         deep: true
       },

app/assets/javascripts/gl_dropdown.js

@@ -512,12 +512,17 @@
 
     // Append the menu into the dropdown
     GitLabDropdown.prototype.appendMenu = function(html) {
+      return this.clearMenu().append(html);
+    };
+
+    GitLabDropdown.prototype.clearMenu = function() {
       var selector;
       selector = '.dropdown-content';
       if (this.dropdown.find(".dropdown-toggle-page").length) {
         selector = ".dropdown-page-one .dropdown-content";
       }
-      return $(selector, this.dropdown).empty().append(html);
+
+      return $(selector, this.dropdown).empty();
     };
 
     GitLabDropdown.prototype.renderItem = function(data, group, index) {

changelogs/unreleased/issue-sidebar-empty-assignee.yml

+---
+title: Resets assignee dropdown when sidebar is open
+merge_request:
+author:

spec/features/boards/sidebar_spec.rb

@@ -141,6 +141,36 @@ describe 'Issue Boards', feature: true, js: true do
         end
       end
     end
+
+    it 'resets assignee dropdown' do
+      page.within(first('.board')) do
+        first('.card').click
+      end
+
+      page.within('.assignee') do
+        click_link 'Edit'
+
+        wait_for_ajax
+
+        page.within('.dropdown-menu-user') do
+          click_link user.name
+
+          wait_for_vue_resource
+        end
+
+        expect(page).to have_content(user.name)
+      end
+
+      page.within(first('.board')) do
+        find('.card:nth-child(2)').click
+      end
+
+      page.within('.assignee') do
+        click_link 'Edit'
+
+        expect(page).not_to have_selector('.is-active')
+      end
+    end
   end
 
   context 'milestone' do