HTML/CSS - My div is just not being sticky and I do not know why

Question

For some reason I cannot understand why this is not working. My only difficulty: The header div is just not being a sticky.

HTML:

<div class="header">
    <div class="header-content">
        <img src="mycoollogo.svg" class="logo" />
    </div>
</div>

CSS:

.logo {
  height: 3rem;
}

.header {
  position: -webkit-sticky; /* Safari Support */
  position: sticky;

  height: 3.5rem;
  margin-top: 0rem;
  margin-left: 0rem;
  margin-right: 0rem;

  background: var(--accent-color);

  color: white;
}

.header-content {
  height: 100%;
  display: flex;
  flex-direction: row;
  align-items: center;

  margin-left: 6rem;
  margin-right: 6rem;
}

Answer 1

your code looks fine the only issue I see is that you should be using top:0 along with the position sticky to make this work.

Here I have modified your code a bit to make it visible use the run snippet tab below and try to scroll, your header should be sticky now.

.logo {
  height: 3rem;
}

.wrapper {
  position: relative;
}

.header {
  position: -webkit-sticky; /* Safari Support */
  position: sticky;
  top: 0;
  height: 3.5rem;
  margin-top: 0rem;
  margin-left: 0rem;
  margin-right: 0rem;

  background: var(--accent-color);

  color: white;
}

.header-content {
  height: 100%;
  display: flex;
  flex-direction: row;
  align-items: center;

  margin-left: 6rem;
  margin-right: 6rem;
}

.top-content {
  background: blue;
  height: 100px;
  position: sticky;
}

.bottom-content {
  background: red;
  height: 5000px;
}

<div class="wrapper">
  <div class="top-content"></div>
  <div class="header">
    <div class="header-content">
        <img src="mycoollogo.svg" class="logo" />
    </div>
</div>
<div class="bottom-content"></div>
</div>