<div class="wrapper">
  <div class="bluebox"></div>
  <!--.redbox内でsticky-->
  <div class="redbox">
    <div class="btn">button</div>
  </div>
  <div class="greenbox"></div>
</div>
.bluebox{
  height:200px;
  background:#bad4ff;
}
.redbox{
  height:500px;
  background:#fdd;
}
.greenbox{
  height:1000px;
  background:#ddffe9;
}
.btn{
  /*↓btnスタイル*/
  width:100px;
  padding:20px 0;
  background:#666;
  color:#fff;
  text-align:center;
  /*↑btnスタイル*/
  position:sticky;
  top:10px;
}

External CSS

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External JavaScript

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