粘性导航在第一次滚动时向上滑动不平滑。 好之后

Question

This is driving me mad and I can't find the cause. I'm using the following to make my nav slide up on scroll down and back on scroll up.

var prevScrollpos = window.pageYOffset;
window.onscroll = function() {
  var currentScrollPos = window.pageYOffset;
  if (prevScrollpos > currentScrollPos) {
    document.getElementById("navbar").style.top = "0";
  } else {
    document.getElementById("navbar").style.top = "-140px";
  }
  prevScrollpos = currentScrollPos;
} 

Here's the CSS

#navbar {
    position: fixed;
    width: 100vw;
    z-index: 99;
    transition: all 0.3s ease-in-out;
    overflow: hidden;
}

The problem is that on the first scroll down after page load it just disappears abruptly. It's only on subsequent scrolls that it slides smoothly.

Answer 1

It was that I hadn't specified the initial position. I'll definitely be going back to rework it based on the advice given.

Thanks again.