Use hidden inputs instead of `data-selected`

app/assets/javascripts/labels_select.js.coffee

@@ -191,19 +191,16 @@ class @LabelsSelect
             callback data
 
         renderRow: (label) ->
-          if $.isArray(selectedLabel)
-            selected = ''
-            $.each selectedLabel, (i, selectedLbl) ->
-              selectedLbl = selectedLbl.trim()
-              if selected is '' and label.title is selectedLbl
-                selected = 'is-active'
-          else
-            selected = if label.title is selectedLabel then 'is-active' else ''
+          selectedClass = ''
+          if $selectbox.find("input[type='hidden']\
+            [name='#{$dropdown.data('field-name')}']\
+            [value='#{label.id}']").length
+            selectedClass = 'is-active'
 
           color = if label.color? then "<span class='dropdown-label-box' style='background-color: #{label.color}'></span>" else ""
 
           "<li>
-            <a href='#' class='#{selected}'>
+            <a href='#' class='#{selectedClass}'>
               #{color}
               #{label.title}
             </a>

app/views/shared/issuable/_sidebar.html.haml

@@ -94,7 +94,7 @@
             - issuable.labels.each do |label|
               = hidden_field_tag "#{issuable.to_ability_name}[label_names][]", label.id, id: nil
             .dropdown
-              %button.dropdown-menu-toggle.js-label-select.js-multiselect{type: "button", data: {toggle: "dropdown", field_name: "#{issuable.to_ability_name}[label_names][]", ability_name: issuable.to_ability_name, show_no: "true", show_any: "true", selected: issuable.label_names.join(","), project_id: (@project.id if @project), issue_update: issuable_json_path(issuable), labels: (namespace_project_labels_path(@project.namespace, @project, :json) if @project)}}
+              %button.dropdown-menu-toggle.js-label-select.js-multiselect{type: "button", data: {toggle: "dropdown", field_name: "#{issuable.to_ability_name}[label_names][]", ability_name: issuable.to_ability_name, show_no: "true", show_any: "true", project_id: (@project.id if @project), issue_update: issuable_json_path(issuable), labels: (namespace_project_labels_path(@project.namespace, @project, :json) if @project)}}
                 %span.dropdown-toggle-text
                   Label
                 = icon('chevron-down')