Renderer.php

<?php

class Renderer {
  …

  // unchanged relative to HEAD
  public function renderRoot(&$elements) {
    return $this->render($elements, TRUE);
  }

  // new!
  public function renderPlain(&$elements) {
    $current_stack = $this->stack;
    $this->stack = NULL;
    $output = $this->render($elements, TRUE);
    $this->stack = $current_stack;
    return $output;
  }

  // and in ::render(), we still have this:
  public function render(…) {
    …
      if ($this->stack->count() !== 1) {
        throw new \LogicException('A stray drupal_render() invocation with $is_root_call = TRUE is causing bubbling of attached assets to break.');
      }
    …
  }

}

::renderRoot() may not be called inside ::renderRoot()

::renderPlain() may be called from inside anything. (Hence tokens and check_markup() can be called from anything — which makes sense.)