.panels {
  display: grid;
  /* The Trick 👇 */
  grid-template-columns: auto 1fr;
}
.resizer {
  overflow: hidden;
  resize: horizontal;
  /* Scale up the handle */
  scale: 4;
  max-width: 100cqi;
  clip-path: inset(
    calc(100% - 12px)
    0
    0
    calc(100% - 12px)
  );
}
      
    

      
<div class="panels">
  <div class="panel">
    <!-- The Magic Element 👇 -->
    <div class="resizer"></div>
  </div>
  <div class="panel"></div>    
</div>
<!--
  The "Trick" is to lean into display: grid.
  Use a relatively positioned resizable element.
  And scale it up so you can grab the handle.
-->