The selected value is lost when validation fails

I'm having the same problem.

This seem to be the same as Widget broken when form validations fails

This doesn't seem to be a duplicate of #1101264: Widget broken when form validations fails which is about missing css/js. This issue is about losing the selected options.

I'm running into this issue right now and am looking for a (possible) solution.

I've made a little fix to get it working. Added an after-build where the html is rebuilt with the correct selection.

Not sure if this is the most optimal way

Patch in #5 worked for me. Had to apply it by hand, but it might be a mistake on my side or because I had already another patch for multiselect.

Thanks!

Thanks for the great work! This has been committed in 8e8bedc.

Just a hint for people who may use this widget within a profile2 entity: profile2 fields are nested into an array named "profile_PROFILE_NAME" within the form_state array. So to make this work I changed the function in the above patch:

 /**
 * After_build callback for this widget.
 */
function _multiselect_after_build($element, $form_state) {
 
  $value_key = $element['#columns'][0];
  $items = array();
  
  // for profile2 fields
  if ($element['#entity_type'] == 'profile2') {
  $profile_array = 'profile_'. $element['#bundle'];
      if (isset($form_state['values'][$profile_array])) {  
          if (isset($form_state['values'][$profile_array][$element['#field_name']][LANGUAGE_NONE])) {  
                foreach ($form_state['values'][$profile_array][$element['#field_name']][LANGUAGE_NONE] as $nid) {
                  $items[] = array($value_key => $nid);
                }
          }  
      }
  } 
  
  // standard for all other fields 
  else {
      if (isset($form_state['values'][$element['#field_name']][LANGUAGE_NONE])) {
        foreach ($form_state['values'][$element['#field_name']][LANGUAGE_NONE] as $nid) {
          $items[] = array($value_key => $nid);
        }
      }
  }

  $widget = _multiselect_build_widget_code($element['#options'], $items, $element, $element['#required']);

  $element['#prefix'] = $widget['prefix_pre'] . $widget['prefix_options'] . $widget['prefix_post'];

  return $element;
}

Note that the suggested patch breaks any hook_form_alter() or hook_field_widget_form_alter() implementations where the #prefix is also modified. As the patch causes the #prefix value to be refilled after the form post.

As this module 'relies' heavily on the form element's #prefix value, I'm unsure how to cleanly fix the addressed problem without being in the way of other modules wanting to add something to the #prefix value.