How to preview the edited content widget ?

I ended up solving this with with a custom module, borrowing lots of code from StepOne.php:

use Drupal\Core\Form\FormStateInterface;
use Drupal\stacks\Widget\WidgetTemplates;
use Drupal\stacks\Entity\WidgetInstanceEntity;

/**
 * Implements hook_form_alter().
 */
function gcu_stacks_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if (isset($form['field_stack_widgets']['widget'])) {
    // Get stacks widgets.
    $widgets = &$form['field_stack_widgets']['widget'];

    foreach ($widgets as $delta => $widget) {
      if (is_array($widget) && array_key_exists('widget_instance_id', $widget)) {
        $widget_instance_id = $widget['widget_instance_id']['#default_value'];
        $widget_instance = WidgetInstanceEntity::load($widget_instance_id);

        if (empty($widget_instance)) {
          continue;
        }

        // Add the preview image.
        $preview_image = gcu_stacks_get_widget_instance_image($widget_instance);
        if (!empty($preview_image)) {
          // Insert the preview image into the form, before the delete button.
          $keys = array_keys($form['field_stack_widgets']['widget'][$delta]['data']);
          $position = array_search('delete', $keys);

          $before = array_slice($form['field_stack_widgets']['widget'][$delta]['data'], 0, $position, TRUE);
          $after = array_slice($form['field_stack_widgets']['widget'][$delta]['data'], $position, NULL, TRUE);

          $before['preview_image'] = ['#markup' => $preview_image];
          $form['field_stack_widgets']['widget'][$delta]['data'] = $before + $after;
        }
      }
    }
  }
}

function gcu_stacks_get_widget_instance_image($widget_instance) {
  $entity = $widget_instance->getWidgetEntity();
  $type = $entity->getType();
  $templates = WidgetTemplates::getTemplatesSelect();
  $base_dir_stacks = WidgetTemplates::templateDir();

  foreach ($templates as $bundle => $bundle_templates) {
    // Continue if this is not the widget type we are looking for.
    if ($bundle != $type) {
      continue;
    }

    foreach ($bundle_templates as $bundle_template => $bundle_template_display) {
      // Continue if this is not the bundle template we are looking for.
      if ($widget_instance->getTemplate() != $bundle_template) {
        continue;
      }

      // Define the preview image.
      $renamed_dir = str_replace(['_', ' '], '-', $bundle);
      $renamed_template = str_replace(['_', ' '], '-', $bundle_template);
      $preview_image_file = DRUPAL_ROOT . '/' . $base_dir_stacks . '/' . $renamed_dir . '/images/' . $renamed_template;
      $preview_images = array_merge(glob($preview_image_file . '.{jpg,jpeg,png,gif}', GLOB_BRACE), glob($preview_image_file . '.{JPG,JPEG,PNG,GIF}', GLOB_BRACE));

      if (!empty($preview_images)) {
        $preview_image_file = substr($preview_images[0], strlen(DRUPAL_ROOT));
        $preview_image_array = [
          '#type' => 'container',
          'img' => [
            '#type' => 'html_tag',
            '#tag' => 'img',
            '#attributes' => [
              'width' => 241,
              'src' => $preview_image_file,
              'title' => $bundle_template_display,
              'class' => ['preview-image'],
            ],
          ],
        ];
      }
      else {
        $preview_image_array = [
          '#type' => 'container',
          'img' => [
            '#type' => 'html_tag',
            '#tag' => 'img',
            '#attributes' => [
              'width' => 241,
              'height' => 234,
              'title' => '',
              'alt' => '',
              'src' => '/' . \Drupal::service('extension.list.module')->getPath('stacks') . '/images/no-preview-img.png',
              'class' => ['preview-image'],
            ],
          ],
        ];
      }

      $preview_image = \Drupal::service('renderer')->render($preview_image_array);

      return $preview_image;
    }
  }
}