public function test_update_filter() {
    $original_css = 'body { color:red; }';
    $post_id      = $this->factory()->post->create(
      array(
        'post_title'   => $this->setting->stylesheet,
        'post_name'    => $this->setting->stylesheet,
        'post_content' => $original_css,
        'post_status'  => 'publish',
        'post_type'    => 'custom_css',
      )
    );

    $overridden_css = 'body { color:green; }';
    $this->wp_customize->set_post_value( $this->setting->id, $overridden_css );

    $post           = get_post( $post_id );
    $original_title = $post->post_title;

    add_filter( 'update_custom_css_data', array( $this, 'filter_update_custom_css_data' ), 10, 3 );
    $this->setting->save();

    $post = get_post( $post_id );
    $this->assertSame( $original_title, $post->post_title );
    $this->assertStringContainsString( $overridden_css, $post->post_content );
    $this->assertStringContainsString( '/* filtered post_content */', $post->post_content );
    $this->assertStringContainsString( '/* filtered post_content_filtered */', $post->post_content_filtered );
  }