Fix GFM dropdown not showing at beginning of new lines

Closes #26145

app/assets/javascripts/gfm_auto_complete.js.es6

@@ -77,7 +77,7 @@
         var _a, _y, regexp, match, atSymbolsWithBar, atSymbolsWithoutBar;
         atSymbolsWithBar = Object.keys(this.app.controllers).join('|');
         atSymbolsWithoutBar = Object.keys(this.app.controllers).join('');
-        subtext = subtext.split(' ').pop();
+        subtext = subtext.split(/\s+/g).pop();
         flag = flag.replace(/[\-\[\]\/\{\}\(\)\*\+\?\.\\\^\$\|]/g, "\\$&");
 
         _a = decodeURI("%C3%80");

changelogs/unreleased/gfm-new-line-fix.yml

+---
+title: Fixed GFM dropdown not showing on new lines
+merge_request:
+author:

spec/features/issues/gfm_autocomplete_spec.rb

@@ -47,6 +47,18 @@ feature 'GFM autocomplete', feature: true, js: true do
       expect_to_wrap(true, label_item, note, label.title)
     end
 
+    it "shows dropdown after a new line" do
+      note = find('#note_note')
+      page.within '.timeline-content-form' do
+        note.native.send_keys('test')
+        note.native.send_keys(:enter)
+        note.native.send_keys(:enter)
+        note.native.send_keys('@')
+      end
+
+      expect(page).to have_selector('.atwho-container')
+    end
+
     it "does not show dropdown when preceded with a special character" do
       note = find('#note_note')
       page.within '.timeline-content-form' do