<!DOCTYPE html>

<style>
.fixed-pos{
  position: fixed;
  background: red;
}

.inner {
  width: 100%;
  background: green;
}

.flexbox {
  display: flex;
}

.column {
  flex-direction: column;
}
</style>

<p>You should see no red.</p>

<div class="fixed-pos" >
  <div class="flexbox column">

    <div class="flexbox">
      <div class="inner">
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
      </div>
    </div>

    <div class="flexbox">
      <div class="inner">
        YYYY
      </div>
    </div>

  </div>
</div>