Fix header menu flash before clicking menu items

Fix header menu flash before clicking menu items

Add the following in header-menu.js to:

deactivate(event) {
  if (!(event.target instanceof Element)) return;
Idea
  // Guard against spurious pointerleave caused by DOM node replacement
  // (e.g. mega-menu content hydrating in via Section Rendering API mid-hover)
  if (event instanceof PointerEvent && event.type === 'pointerleave') {
    const atPoint = document.elementFromPoint(event.clientX, event.clientY);
    if (atPoint && this.contains(atPoint)) return;
  }
  const menu = findSubmenu(this.#state.activeItem);
  // ...rest unchanged
}