Support non-ASCII characters in GFM autocomplete

app/assets/javascripts/gfm_auto_complete.js.es6

@@ -83,12 +83,12 @@
         _a = decodeURI("%C3%80");
         _y = decodeURI("%C3%BF");
 
-        regexp = new RegExp("^(?:\\B|[^a-zA-Z0-9_" + atSymbolsWithoutBar + "]|\\s)" + flag + "(?![" + atSymbolsWithBar + "])([A-Za-z" + _a + "-" + _y + "0-9_\'\.\+\-]*)$", 'gi');
+        regexp = new RegExp("^(?:\\B|[^a-zA-Z0-9_" + atSymbolsWithoutBar + "]|\\s)" + flag + "(?![" + atSymbolsWithBar + "])(([A-Za-z" + _a + "-" + _y + "0-9_\'\.\+\-]|[^\\x00-\\x7a])*)$", 'gi');
 
         match = regexp.exec(subtext);
 
         if (match) {
-          return match[2] || match[1];
+          return (match[1] || match[1] === "") ? match[1] : match[2];
         } else {
           return null;
         }

changelogs/unreleased/27067-mention-user-dropdown-does-not-suggest-by-non-ascii-characters-in-name.yml

+---
+title: Support non-ASCII characters in GFM autocomplete
+merge_request: 8729
+author:

spec/features/issues/gfm_autocomplete_spec.rb

@@ -2,7 +2,7 @@ require 'rails_helper'
 
 feature 'GFM autocomplete', feature: true, js: true do
   include WaitForAjax
-  let(:user)    { create(:user, username: 'someone.special') }
+  let(:user)    { create(:user, name: '💃speciąl someone💃', username: 'someone.special') }
   let(:project) { create(:project) }
   let(:label) { create(:label, project: project, title: 'special+') }
   let(:issue)   { create(:issue, project: project) }
@@ -59,6 +59,19 @@ feature 'GFM autocomplete', feature: true, js: true do
     expect(find('#at-view-64')).to have_selector('.cur:first-of-type')
   end
 
+  it 'includes items for assignee dropdowns with non-ASCII characters in name' do
+    page.within '.timeline-content-form' do
+      find('#note_note').native.send_keys('')
+      find('#note_note').native.send_keys("@#{user.name[0...8]}")
+    end
+
+    expect(page).to have_selector('.atwho-container')
+
+    wait_for_ajax
+
+    expect(find('#at-view-64')).to have_content(user.name)
+  end
+
   it 'selects the first item for non-assignee dropdowns if a query is entered' do
     page.within '.timeline-content-form' do
       find('#note_note').native.send_keys('')