getLastPreviewRequestTransferSize example

$this->waitForEditor();
    $assert_session = $this->assertSession();
    $this->assertNotEmpty($assert_session->waitForElementVisible('css', 'img[src*="image-test.png"]'));
    $element = $assert_session->elementExists('css', '[data-media-embed-test-active-theme]');
    $this->assertSame('stable9', $element->getAttribute('data-media-embed-test-active-theme'));
    // Assert that the first preview request transferred >500 B over the wire.     // Then toggle source mode on and off. This causes the CKEditor widget to be     // destroyed and then reconstructed. Assert that during this reconstruction,     // a second request is sent. This second request should have transferred 0     // bytes: the browser should have cached the response, thus resulting in a     // much better user experience.     $this->assertGreaterThan(500, $this->getLastPreviewRequestTransferSize());
    $this->pressEditorButton('Source');
    $this->assertNotEmpty($assert_session->waitForElement('css', '.ck-source-editing-area'));
    // CKEditor 5 is very smart: if no changes were made in the Source Editing     // Area, it will not rerender the contents. In this test, we     // want to verify that Media preview responses are cached on the client side     // so it is essential that rerendering occurs. To achieve this, we append a     // single space.     $source_text_area = $this->getSession()->getPage()->find('css', '[name="body[0][value]"] + .ck-editor textarea');
    $source_text_area->setValue($source_text_area->getValue() . ' ');
    $this->pressEditorButton('Source');
    $this->assertNotEmpty($assert_session->waitForElementVisible('css', 'img[src*="image-test.png"]'));