JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent" style="display:inline-block;">
  <div class="flex">
    <div class="flex-child">One</div>
    <div class="flex-child">Two</div>
    <div class="flex-child">Three (really really long)</div>
    <div class="flex-child">Four</div>
  </div>
</div>
<br>
<div class="parent" style="float:left;">
  <div class="flex">
    <div class="flex-child">One</div>
    <div class="flex-child">Two</div>
    <div class="flex-child">Three (really really long)</div>
    <div class="flex-child">Four</div>
  </div>
</div>

CSS

.parent {
  padding: 10px;
  background: red;
}

.flex {
  display: flex;
}

.flex-child {
  flex: 1 0 100px;
  /*width: 100px;  replace the flex-basis by with to solve the issue */
  background: #EEE;
  border: 1px solid black;
  white-space: nowrap;
  overflow: hidden;
}