htmx:beforeSwap fails with TypeError requestAssetsLoaded.get(...) is undefined

Problem/Motivation

Looks like htmx:beforeRequest is never triggered, and requestAssetsLoaded isn't populated, causing htmx:beforeSwap to fail with Uncaught TypeError: can't access property "then", requestAssetsLoaded.get(...) is undefined.

Steps to reproduce

Add the following controller:

# htmx_example.routing.yml
htmx_example.htmx_response:
  path: '/htmx-example/htmx'
  defaults:
    _controller: '\Drupal\htmx_example\Controller\HtmxExampleController::htmx'
  options:
    _htmx_route: 'TRUE'
  requirements:
    _permission: 'access content'

# src/Controller/HtmlExampleController.php

declare(strict_types=1);

namespace Drupal\htmx_example\Controller;

use Drupal\Core\Controller\ControllerBase;
use Drupal\Core\Htmx\Htmx;
use Drupal\Core\Url;

final class HtmxExampleController extends ControllerBase {

  public function htmx(): array {
    return [
      '#type' => 'markup',
      '#markup' => 'Some content',
    ];
  }

}

and a render array like:

$htmx = new Htmx();
$htmx->get(new Url('htmx_example.htmx_response'))
  ->trigger('load');

$build = [
  '#type' => 'html_tag',
  '#tag' => 'div',
];
$htmx->applyTo($build);

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet