滚动恢复时Header引发内容跳动，求平滑过渡解决方案

我查阅了诸多关于经典Header跳动问题的帖子，但未找到能解决我问题的方案。我实现了一个向下滚动时收缩的Header，搭配navbar_placeholder可正常维持Header与内容的间距；但向上滚动恢复时，内容会突然跳动，Header直接恢复原尺寸，我希望实现平滑过渡效果，却无法定位问题根源。

HTML 代码 #

<!DOCTYPE html>
<html lang="de">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>WeSoDev</title>
    <link rel="stylesheet" href="index.css">
</head>
<body>
<div id="navbar">
    <nav id="nav">
      <h1 id="header">header</h1>
    </nav>
</div>
<div id="navbar_placeholder"></div>
<div id="content">
    <h1 class="text1">Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.
        Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis at vero eros et accumsan et iusto odio dignissim qui blandit praesent luptatum zzril delenit augue duis dolore te feugait nulla facilisi. Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat.
        Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat. Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis at vero eros et accumsan et iusto odio dignissim qui blandit praesent luptatum zzril delenit augue duis dolore te feugait nulla facilisi.
        Nam liber tempor cum soluta nobis eleifend option congue nihil imperdiet doming id quod mazim placerat facer possim assum. Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat. Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat.
        Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis.
        At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.</h1>
</div>
    <script src="index.js"></script>
</body>
</html>

CSS 代码 #

:root{
    --background-color: #001728;
    --darker-background-color: #000000;
    --accent-color: #20cc5b;
    --text-color: #FFFFFF;
    --navbar-height: 80px;
}
html{
    height: 100%;
}
body{
    height: 100%;
    background-color: var(--background-color);
}
nav{
    height: var(--navbar-height);
    background-color: red;
    display: flex;
    justify-content: space-between;
    align-items: center;
    border-bottom: 4px solid var(--accent-color);
    transition-property: height;
    transition-duration: 0.5s;
}
.navbar_placeholder {
    height: var(--navbar-height);
    display: none;
    transition: height 0.5s;
}
.text1{
    padding: 30px;
  color: white;
}
.sticky {
    position: fixed;
    top: 0;
    width: 100%;
}
.disappear {
    opacity: 0%;
    transition-duration: 0.5s;

}
.shrink {
    height: 40px;
    transition-property: height;
    transition-duration: 0.5s;
}
.display {
    display: block;
    height: var(--navbar-height);
}

JavaScript 代码 #

var content = document.getElementById("content");
var nav = document.getElementById("nav");
var placeholder = document.getElementById("navbar_placeholder");
var sticky = navbar.offsetTop;

window.onscroll = function() {myFunction()};
function myFunction() {
    if (window.pageYOffset > sticky) {
        navbar.classList.add("sticky");
        placeholder.classList.add("display");
        nav.classList.add("shrink");
        content.classList.add("content");
    } else {
        navbar.classList.remove("sticky");
        placeholder.classList.remove("display");
        nav.classList.remove("shrink");
        content.classList.remove("content");
    }
}

内容的提问来源于stack exchange，提问作者Marcii