如何拥有一个固定的导航栏，当你向下滚动时不会改变大小？

.navbar {
  position: fixed;
  top: 0;
  width: 100%;
  height: 60px;
  background-color: #f1f1f1;
}

window.addEventListener('scroll', function() {
  var navbar = document.querySelector('.navbar');
  var scrollPosition = window.pageYOffset || document.documentElement.scrollTop;

  if (scrollPosition >= navbar.offsetHeight) {
    navbar.classList.add('fixed');
  } else {
    navbar.classList.remove('fixed');
  }
});

.navbar.fixed {
  position: fixed;
  top: 0;
}