why the height of my sticky navbar disappear when I scroll?

Question

I'm trying to build a website with a sticky nav bar in JavaScript. For the first load of the page everything is okay. But when I scroll, the navbar is flickering and after this the body up (see the pictures). I don't know why.

Just after the navbar I have slideshow and because of this the pictures are cut by the navbar and I'm on the top of the page.

See my code too below...

/*sticky_navbar*/

window.onscroll = function() {
  myFunction()
};

var navbar = document.getElementById("header");
var sticky = navbar.offsetTop;

function myFunction() {
  if (window.pageYOffset >= sticky) {
    navbar.classList.add("sticky")
  } else {
    navbar.classList.remove("sticky");
  }
}

@charset "UTF-8";

/* CSS Document */

body {
  margin: 0;
  font-size: 28px;
  background-color: #00011f;
  display: flex;
  flex-direction: column;
  margin: auto;
}


/* Style the navbar */

#header {
  display: flex;
  justify-content: flex-end;
  background: rgba(139, 139, 157, 1);
  z-index: 2;
}

#Title {
  margin: 0 auto 0 0;
  height: 20px;
  margin-top: 20px;
  padding-left: 10px;
  border-bottom: 1px solid white;
  padding-top: 10px;
  padding-bottom: 35px;
  flex: 1;
}

#navbar {
  overflow: hidden;
  background: rgba(139, 139, 157, 0);
  display: flex;
  justify-content: flex-end;
  border-bottom: 5px solid white;
  padding-bottom: 20px;
  padding-top: 20px;
}

.menu:nth-child(1) {
  order: 1;
}

.menu:nth-child(2) {
  order: 4;
}

.menu:nth-child(3) {
  order: 3;
}

.menu:nth-child(4) {
  order: 2;
}

.menu:nth-child(5) {
  order: 5;
}

IMG.background {
  display: block;
  margin-left: auto;
  margin-right: auto;
  z-index: 1;
  width: 60%;
}

#navbar a {
  display: block;
  color: #FFF;
  text-align: center;
  padding: 10px 16px;
  text-decoration: none;
  font-size: 17px;
}

#navbar a:hover {
  background-color: #ddd;
  color: black;
}

#navbar a.active {
  background: rgba(217, 78, 68, 0.5);
  color: white;
}

.content {
  padding: 16px;
  color: #ddd;
  background-color: #FFF
}

.sticky {
  position: fixed;
  top: 0;
  width: 100%;
}

.sticky+.content {
  padding-top: 60px;
}


/*END NAVBAR*/

#display {
  display: flex;
}

<div id="header">
  <div id="Title">
    <a href="Accueil"><img src="IMAGES/PNG/logo.png" alt="logo" /></a>
  </div>
  <div id="navbar">
    <div class="menu"> <a class="active" href="javascript:void(0)">Blog</a></div>
    <div class="menu"> <a href="blog">Contact</a></div>
    <div class="menu"> <a href="blog">L'électrophotonique</a></div>
    <div class="menu"> <a href="blog">Qui sommes nous?</a></div>
  </div>
</div>
<div class="content">
  <h3>Sticky Navigation Example</h3>
</div>

Answer 1

the body jump to another position because your navbar has height of at least 86px but you gave .sticky+.content only padding-top of 60px.

maybe you can use position: sticky; instead? https://caniuse.com/#feat=css-sticky

or

to prevent flickering give the navbar onload the position fixed

#navbar {
  overflow: hidden;
  background: rgba(139, 139, 157, 0);
  display: flex;
  justify-content: flex-end;
  border-bottom: 5px solid white;
  padding-bottom: 20px;
  padding-top: 20px;
  position: fixed;
  top: 0;
  width: 100%;
}
.content {
  padding: 60px 16px 16px 16px;
  color: #ddd;
  background-color: #FFF
}