<div class="box">
  <div class="header">Заголовок</div>
  <div class="inner">Текст</div>
  <div class="inner">Текст</div>
  <div class="inner">Текст</div>
  <div class="inner">Текст</div>
  <div class="inner">Текст</div>
  <div class="inner">Текст</div>
  <div class="inner">Текст</div>
  <div class="inner">Текст</div>
  
</div>

.box{
  width: 500px;
  height: 300px;
  overflow: scroll;
  position: relative;
  }
.box > div {
  margin-bottom: 10px;
  padding: 15px 10px;

}
.header{
  position: sticky;
  top: 0;z-index: 2;
  background: #fff;
  z-index: 100;
}
.header::before{
  content:'';
  display: block;
  top: 0;
  left: 0;
  height: 100%;
  width: calc(100% - 6px);
  position: absolute;
  background: white;
  z-index: -1;
   border: 3px solid black;
  border-radius: 15px;
}

.inner{
  
   border: 3px solid black;
  border-radius: 15px;
}