How can I make a container resizable?
In the official documentation

says the following:


  "#resizable": Controls whether the text area is resizable. Allowed values are "none", "vertical", "horizontal", or "both" (defaults to "vertical").


in this documentation we can see, that this property "#resizable" is applicable only to textarea.

Then, you can try: 

function test_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  if ($form_id == 'node_page_edit_form' || $form_id == 'node_page_form') {
    $form['body']['widget']['0']['#resizable']='horizontal';

    //or
    //$form['body']['widget']['0']['#attributes']["style"]="resize:vertical";

  }
}


if you change the text format in "restricted HTML" you will see



if you want to use the "both" parameter

function test_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  if ($form_id == 'node_page_edit_form' || $form_id == 'node_page_form') {
      $form['body']['widget']['0']['#resizable']='both';

  }
}


if you change the text format in "restricted HTML" you will see: