Retrigger at.js for slash commands with autocomplete suffix

app/assets/javascripts/gfm_auto_complete.js.es6

@@ -103,6 +103,9 @@
       this.input.each((i, input) => {
         const $input = $(input);
         $input.off('focus.setupAtWho').on('focus.setupAtWho', this.setupAtWho.bind(this, $input));
+        // This triggers at.js again
+        // Needed for slash commands with suffixes (ex: /label ~)
+        $input.on('inserted-commands.atwho', $input.trigger.bind($input, 'keyup'));
       });
     },
     setupAtWho: function($input) {

changelogs/unreleased/27883-autocomplete-seems-to-not-trigger-when-at-character-is-part-of-an-autocompleted-text.yml

+---
+title: Trigger autocomplete after selecting a slash command
+merge_request: 9117
+author:

spec/features/issues/gfm_autocomplete_spec.rb

@@ -182,6 +182,20 @@ feature 'GFM autocomplete', feature: true, js: true do
       expect(page).not_to have_selector('.atwho-view')
     end
 
+    it 'triggers autocomplete after selecting a slash command' do
+      note = find('#note_note')
+      page.within '.timeline-content-form' do
+        note.native.send_keys('')
+        note.native.send_keys('/as')
+        note.click
+      end
+
+      find('.atwho-view li', text: '/assign').native.send_keys(:tab)
+
+      user_item = find('.atwho-view li', text: user.username)
+      expect(user_item).to have_content(user.username)
+    end
+
     def expect_to_wrap(should_wrap, item, note, value)
       expect(item).to have_content(value)
       expect(item).not_to have_content("\"#{value}\"")