Fix overflowing width at mention container

MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/54377

app/assets/stylesheets/vendors/atwho.scss

 .atwho-view {
   overflow-y: auto;
   overflow-x: hidden;
+  max-width: calc(100% - 6px);
 
   .name,
   small.aliases,
@@ -80,10 +81,6 @@
 }
 
 @include media-breakpoint-down(xs) {
-  .atwho-view-ul {
-    width: 350px;
-  }
-
   .atwho-view ul li {
     overflow: hidden;
     text-overflow: ellipsis;

changelogs/unreleased/fix-overflowing-width-at-mention-selector.yml

+---
+title: Fix overflowing width - at mention container
+merge_request: 54377
+author:
+type: fixed