<!DOCTYPE html>
<style>
.flexbox {
    display: flex;
}
.inline-flexbox {
    display: inline-flex;
}
.flex-one {
    flex: 1;
}
.inline-block { display: inline-block; }
.flexbox, .inline-flexbox { background-color: lightgrey; }
.border { border: 11px solid pink; }
.padding { padding: 13px; }
.margin { margin: 8px 0; }
.flexbox > div {
    min-width: 0;
    min-height: 0;
}
</style>

<div>
Should align with the bottom
<div class="inline-block border margin padding" style="background-color: pink">
  <div class="flexbox border margin padding" style="width: 100px; height: 100px; background-color: pink">
      <div style="width: 200px; overflow: scroll; background-color: lightgrey; margin: 10px 0px; border-top: 10px solid pink;"></div>
  </div>
</div>
of the horizontal scrollbar.
</div>

<div>
Should align 10px below the
<div class="inline-block" style="background-color: pink">
  <div class="flexbox" style="width: 100px; height: 100px; background-color: pink">
      <div style="width: 200px; overflow: scroll; background-color: lightgrey; padding-bottom: 10px; margin: 10px 0px; border-top: 10px solid pink; border-bottom: 10px solid pink;"></div>
  </div>
</div>
horizontal scrollbar, if one is visible.
</div>