$form_state->setErrorByName() for a form in a block doesn't work properly

Question

I created a very simple module 'testform'. In this module, I generate a form and a block. (via the drush console)

The form contains a textfield ("housename") and a submit button.

In the form Class, I add a condition in the validate handler for display an error when the user submit the form without any data in the textfield.

The form from the form page (no problem)

1. I load the form page (url path found in the testform.routing.yml)
2. I click to the submit button without enter any data in the housename textfield
3. The next page load, the empty housename field is red and there is an error message "the housename field is empty" => By this way, all is ok

The form from the block (there is the problem)

1. I load a page where is displayed the form block
2. In the block, I click to the sumbit button without enter any data in the textfield
3. The next page load, the empty housename field is red and there isn't any error message

4. If I reload the page, the setError message appears

I don't understand why when the form is displayed in a block, the error message doesn't work properly.

This is the code I am using.

The modules/custom/testform/src/Form/DefaultForm.php file

<?php

namespace Drupal\testform\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Class DefaultForm.
 *
 * @package Drupal\testform\Form
 */
class DefaultForm extends FormBase {


  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'default_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['housename'] = [
      '#type' => 'textfield',
      '#title' => $this->t('housename'),
      '#maxlength' => 64,
      '#size' => 64,
    ];

    $form['submit'] = [
        '#type' => 'submit',
        '#value' => t('Submit'),
    ];

    return $form;
  }

  /**
    * {@inheritdoc}
    */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    $housename = $form_state->getValue('housename');
    if(empty($housename)){
      $form_state->setErrorByName("housename","The housename field is empty");
    }
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // Display result.
    ksm("Work...");

  }

}

The modules/custom/testform/src/Plugin/Block/DefaultBlock.php file

<?php

namespace Drupal\testform\Plugin\Block;

use Drupal\Core\Block\BlockBase;

/**
 * Provides a 'DefaultBlock' block.
 *
 * @Block(
 *  id = "default_block",
 *  admin_label = @Translation("Default block"),
 * )
 */
class DefaultBlock extends BlockBase {

  /**
   * {@inheritdoc}
   */
  public function build() {
    return \Drupal::formBuilder()->getForm('Drupal\testform\Form\DefaultForm');

  }

}

Answer 1

This is a known bug. You can see the same problem with the core user login block upon submitting bad credentials. The comments in that issue note that it's a side effect of another issue which has a fix committed to the 8.2.x branch.