The view preview section sends a POST request, but REST exports routes only accept GET requests

@Rares Badita, thanks for the issue.

Issues are solve on the main development branch first, 11.x. Also restoring the template so we can keep track of any work done here.

I've tinkered a bit with the issue, and I think the behaviour used for processing the form when contextual filter are present should be also used when there are no contextual filters.

Given this is a user-facing PHP error (and the preview just doesn't work in this case), bumping to major.

The issue arises when creating a URL from a request using the Url::createFromRequest($request) method.
When the Router class tries to match the url of the $request parameter to a route, we reach this code:

// Check HTTP method requirement.
if ($requiredMethods = $route->getMethods()) {
  // HEAD and GET are equivalent as per RFC.
  if ('HEAD' === $method = $this->context->getMethod()) {
    $method = 'GET';
  }

  if (!in_array($method, $requiredMethods)) {
    $this->allow = array_merge($this->allow, $requiredMethods);
    $routes->remove($name);
    continue;
  }
}

The problem is that the $method used to check against $requiredMethods is not obtained from the current $request, but from $context, which is the original request from the request stack.

This leads to an issue with REST exports that use the Facets serializer view style. When building the facets results, FacetsUrlGenerator::getUrlForRequest() wants to create a URL from a request to the actual view display(made with GET), not to the preview form(made with POST), thus resulting in a method mismatch and an exception being thrown.

I've added a patch for the Facets module to catch this exception.

I've also changed the issue project from views-ui to facets.