Autocomplete should not create extra list items

When selecting an item in an autocomplete by typing
“Enter”, if you’re in a markdown list item, another
list item should not be created

app/assets/javascripts/lib/utils/text_markdown.js

@@ -367,6 +367,8 @@ function handleContinueList(e, textArea) {
 export function keypressNoteText(e) {
   const textArea = this;
 
+  if ($(textArea).atwho?.('isSelecting')) return;
+
   handleContinueList(e, textArea);
   handleSurroundSelectedText(e, textArea);
 }

spec/features/issues/gfm_autocomplete_spec.rb

@@ -420,6 +420,14 @@ RSpec.describe 'GFM autocomplete', :js do
         end
       end
     end
+
+    context 'when typing enter for autocomplete in a markdown list' do
+      it 'does not create a new list item' do
+        fill_in 'Comment', with: "- @#{user.username}\n"
+
+        expect(find_field('Comment').value).to eq "- @#{user.username}\n"
+      end
+    end
   end
 
   private

spec/frontend/lib/utils/text_markdown_spec.js

+import $ from 'jquery';
 import { insertMarkdownText, keypressNoteText } from '~/lib/utils/text_markdown';
+import '~/lib/utils/jquery_at_who';
 
 describe('init markdown', () => {
   let textArea;
@@ -223,6 +225,24 @@ describe('init markdown', () => {
           expect(textArea.selectionEnd).toBe(text.length);
         });
 
+        // test that when we're in the middle of autocomplete, we don't
+        // add a new list item
+        it.each`
+          text          | expected          | atwho_selecting
+          ${'- item @'} | ${'- item @'}     | ${true}
+          ${'- item @'} | ${'- item @\n- '} | ${false}
+        `('behaves correctly during autocomplete', ({ text, expected, atwho_selecting }) => {
+          jest.spyOn($.fn, 'atwho').mockReturnValue(atwho_selecting);
+
+          textArea.value = text;
+          textArea.setSelectionRange(text.length, text.length);
+
+          textArea.addEventListener('keydown', keypressNoteText);
+          textArea.dispatchEvent(enterEvent);
+
+          expect(textArea.value).toEqual(expected);
+        });
+
         it('does nothing if feature flag disabled', () => {
           gon.features = { markdownContinueLists: false };