chrome/browser/resources/tab_strip/playground/split_tab_playground.html

<style>
  :host {
    display: flex;
    gap: 4px;
    border: 2px dashed var(--google-grey-500);
    border-radius: 12px;
    padding: 4px;
    min-width: fit-content;
    height: 100%;
    flex-direction: column;
  }
</style>

<div id="container" draggable="true">
  <slot id="slot"></slot>
</div>