<div class="content">
    <div class="block"></div>
    <div class="block"></div>
    <div class="block"></div>
    <div class="block fix"></div>
    <div class="block"></div>
    <div class="block"></div>
    <div class="block"></div>
    <div class="block"></div>
    <div class="block"></div>
    <div class="block"></div>
</div>

.block {
    height: auto;
    min-height: 400px;
    border: 2px solid red;
    margin: 20px;
}
.fix {
    background: blue;
    position: sticky;
    top: 0;
}