Fix flaky spec in spec/features/issues/gfm_autocomplete_spec.rb

"GFM autocomplete when tribute_autocomplete feature flag is on
assignees lists users who are currently not assigned to the
issue when using /assign on the second line" was failing

app/assets/javascripts/vue_shared/components/gl_mentions.vue

@@ -43,7 +43,12 @@ const autoCompleteMap = {
       const fullText = this.$slots.default?.[0]?.elm?.value;
       const selectionStart = this.$slots.default?.[0]?.elm?.selectionStart;
 
-      if (!this.assignees) {
+      // Need to check whether sidebar store assignees has been updated
+      // in the case where the assignees AJAX response comes after the user does @ autocomplete
+      const isAssigneesLengthSame =
+        this.assignees?.length === SidebarMediator.singleton?.store?.assignees?.length;
+
+      if (!this.assignees || !isAssigneesLengthSame) {
         this.assignees =
           SidebarMediator.singleton?.store?.assignees?.map(assignee => assignee.username) || [];
       }