<style>
.multicol-a {
  width: 300px;
  column-width: 100px;
  column-gap: 0;
  height: 100px;
}
.multicol-b {
  border: 1px solid silver;
  width: 200px;
  column-width: 51px;
  column-gap: 0;
  height: 50px;
}

.step {
  height: 1px;
}
.float-L {
  width: 1px;
  height: 1px;
  float: left;
}
.float-R {
  width: 1px;
  height: 26px; /* 25 -> 26 crash */
}
</style>

<div class="multicol-a">
  <div class="float-R"></div>
  <div class="multicol-b">
    <div class="step"></div>
    <div>
      <div class="float-L"></div>
    <div>
  </div>
</div>