[解決済み] 私の位置：フレックスボックスを使用すると、スティッキー要素がスティッキーにならない

.flexbox-wrapper {
  display: flex;
  height: 600px;
}
.regular {
  background-color: blue;
}
.sticky {
  position: -webkit-sticky;
  position: sticky;
  top: 0;
  background-color: red;
}

<div class="flexbox-wrapper">
  <div class="regular">
    This is the regular box
  </div>
  <div class="sticky">
    This is the sticky box
  </div>
</div>

.flexbox-wrapper {
  display: flex;
  overflow: auto;
  height: 200px;          /* Not necessary -- for example only */
}
.regular {
  background-color: blue; /* Not necessary -- for example only */
  height: 600px;          /* Not necessary -- for example only */
}
.sticky {
  position: -webkit-sticky; /* for Safari */
  position: sticky;
  top: 0;
  align-self: flex-start; /* <-- this is the fix */
  background-color: red;  /* Not necessary -- for example only */
}

<div class="flexbox-wrapper">
  <div class="regular">
    This is the regular box
  </div>
  <div class="sticky">
    This is the sticky box
  </div>
</div>