LayoutTests/fast/block/float/float-with-negative-margin-formatting-context-root.html

<style>
.float {
  float: left;
  width: 100%;
  height: 50px;
  background-color: green;
}
.flex {
  display: flex;
  margin-left: -1px;
}
</style>
<div style="width: 100px;">
  <div class=float></div>
  <div class=flex>PASS if this text is below the green box</div>
</div>