assertMessagesDisplayed example

$this->drupalLogin($this->drupalCreateUser([
      'access contextual links',
      'configure any layout',
    ]));
    $this->drupalGet('node/1/layout');
    $page->findLink('Add block')->click();
    $this->assertNotEmpty($assert_session->waitForElementVisible('css', '#drupal-off-canvas .block-categories'));
    $page->findLink('Powered by Drupal')->click();
    $this->assertNotEmpty($assert_session->waitForElementVisible('css', '#drupal-off-canvas [name="settings[label]"]'));
    $page->findField('Title')->setValue('');
    $page->findButton('Add block')->click();
    $this->assertMessagesDisplayed();
    $page->findField('Title')->setValue('New title');
    $page->pressButton('Add block');
    $block_css_locator = '#layout-builder .block-system-powered-by-block';
    $this->assertNotEmpty($assert_session->waitForElementVisible('css', $block_css_locator));

    $assert_session->assertNoElementAfterWait('css', '#drupal-off-canvas');
    $assert_session->assertWaitOnAjaxRequest();
    $this->drupalGet($this->getUrl());
    $page->findButton('Save layout')->click();
    $this->assertNotEmpty($assert_session->waitForElement('css', 'div:contains("The layout override has been saved")'));