discourse/plugins/chat/assets/stylesheets/common/chat-side-panel-resizer.scss

.chat-side-panel-resizer {
  top: 0;
  bottom: 0;
  position: absolute;
  z-index: z("composer", "content") - 1;
  transition: background-color 0.15s 0.15s;
  background-color: transparent;

  .touch & {
    left: -6px;
    width: 10px;

    &:active {
      cursor: col-resize;
      background: var(--tertiary);
    }
  }

  .no-touch & {
    left: -3px;
    width: 5px;

    &:hover,
    &:active {
      cursor: col-resize;
      background: var(--tertiary);
    }
  }
}