/**
 * `true` when the element is considered disabled — either via a direct
 * `disabled` attribute, an `aria-disabled="true"` attribute, or an ancestor
 * `<fieldset disabled>` (which disables all form controls within it, per
 * HTML spec).
 *
 * A `null` element is treated as disabled: callers typically use this to
 * guard keyboard navigation, and a missing element should never receive
 * focus.
 */
export function isElementDisabled(element: Element | null): boolean {
	if (element == null) {
		return true;
	}

	if (element.hasAttribute("disabled")) {
		return true;
	}

	if (element.getAttribute("aria-disabled") === "true") {
		return true;
	}

	// Nearest ancestor <fieldset disabled> disables all contained form
	// controls. We walk up the tree rather than relying on :disabled because
	// the CSS pseudo-class does not work via `element.matches` in every
	// environment (notably happy-dom).
	let parent: Element | null = element.parentElement;
	while (parent != null) {
		if (
			parent.tagName === "FIELDSET" &&
			(parent as HTMLFieldSetElement).disabled
		) {
			return true;
		}
		parent = parent.parentElement;
	}

	return false;
}