Return only main content for endpoints designed to service htmx requests

Test passes. CR created.

I looked at \Drupal\Core\Routing\RequestFormatRouteFilter and _format seems more about matching a requested format with routes that serve that format. I'm looking for something definitive on the route rather than dependent on the request.

From Structure of routes:

_format: Use this to check the type of the request. For example, you can have _format: json and only match requests where the '_format' query parameter is 'json'.

The intent of this issue is to be able to designate in a route definition that the route should always render its responses using HtmxRenderer. There is no dependency on the request. Also, the format for responses from HTMX requests is HTML.

We already see the need for such routes in the experimentation @nod_ is doing with Views. The wrapper format query parameter is extra baggage in filtered views along with all the filter parameters.

In the contrib module I have a dedicated View display that has a path. In ::getRoute I explicitly set the _htmx_route option. If we could do that based on the "use ajax" checkbox then none of the HTMX requests added in the view would need to add the wrapper format query parameter.

Thanks for the +1 on the route option!

With the display having it's dyanmic route method, this route option was an advantage in the contrib module:

  /**
   * {@inheritdoc}
   */
  protected function getRoute($view_id, $display_id) {
    $route = parent::getRoute($view_id, $display_id);

    // Explicitly set HTML as the format for Page displays.
    $route->setRequirement('_format', 'html');

    if ($this->getOption('use_admin_theme')) {
      $route->setOption('_admin_route', TRUE);
    }

    // This is an HTMX route.
    $route->setOption('_htmx_route', TRUE);

    return $route;
  }

All tests passing after applying suggestions.

Setting back to RTBC per #20