Fixes #2890 - Auto-Complete on Typing for Tags is hidden behind Pop-Up Windows.

app/assets/stylesheets/zammad.scss

@@ -7042,7 +7042,7 @@ footer {
 
 /* allow/show autocomplete in modal dialog */
 .ui-autocomplete.ui-widget-content {
-  @extend .zIndex-8;
+  @extend .zIndex-9;
   position: absolute;
   max-height: 380px;
   overflow: auto;

spec/system/manage/trigger_spec.rb

@@ -0,0 +1,28 @@
+require 'rails_helper'
+
+RSpec.describe 'Manage > Trigger', type: :system do
+
+  context 'Perform' do
+
+    context 'Tags' do
+
+      it 'shows tag selection list in foreground' do
+        tag_item = create :tag_item
+
+        visit '/#manage/trigger'
+        click '.page-header-meta .btn--success'
+
+        modal_ready
+
+        within '.modal .ticket_perform_action' do
+          find('.js-attributeSelector select').select('Tags')
+
+          input = find('.js-value .token-input')
+          input.fill_in with: tag_item.name.slice(0, 3)
+        end
+
+        expect(page).to have_css('.ui-autocomplete.ui-widget-content') { |elem| !elem.obscured? }
+      end
+    end
+  end
+end