Enhancement: Allow selection of multiple values in Trigger custom select attribute filter. Closes #1559.

app/assets/javascripts/app/controllers/_ui_element/ticket_selector.coffee

@@ -87,6 +87,9 @@ class App.UiElement.ticket_selector
               config.operator = operator
             elements["#{groupKey}.#{config.name}"] = config
 
+          if config.tag == 'select'
+            config.multiple = true
+
     if attribute.out_of_office
       elements['ticket.out_of_office_replacement_id'] =
         name: 'out_of_office_replacement_id'

spec/system/admin/trigger/custom_attribute_as_multiselect_spec.rb

@@ -0,0 +1,29 @@
+require 'rails_helper'
+
+RSpec.describe 'Admin Panel > Trigger', type: :system do
+  it 'custom select attribute allows to select multiple values', db_strategy: :reset do
+    attribute = create_attribute :object_manager_attribute_select,
+                                 data_option: {
+                                   options:    {
+                                     'name 1': 'name 1',
+                                     'name 2': 'name 2',
+                                   },
+                                   default:    '',
+                                   null:       false,
+                                   relation:   '',
+                                   maxlength:  255,
+                                   nulloption: true,
+                                 }
+
+    visit '/#manage/trigger'
+    click '.page-header-meta .btn--success'
+
+    modal_ready
+
+    within '.modal .ticket_selector' do
+      find('.js-attributeSelector select').select(attribute.display)
+
+      expect(find('.js-value select')).to be_multiple
+    end
+  end
+end