Fix selectbox when submit MR from fork to origin

app/controllers/projects/merge_requests_controller.rb

@@ -152,6 +152,10 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     @target_project = selected_target_project
     @target_branches = @target_project.repository.branch_names
     @target_branches
+
+    respond_to do |format|
+      format.js
+    end
   end
 
   def ci_status

app/views/projects/merge_requests/update_branches.js.haml

 :plain
   $(".target_branch").html("#{escape_javascript(options_for_select(@target_branches))}");
-  $(".target_branch").trigger("select2:updated");
+
+  $('select.target_branch').select2({
+    width: 'resolve',
+    dropdownAutoWidth: true
+  });
+
   $(".mr_target_commit").html("");
-  $(".target_branch").trigger("change");