opencode/packages/ui/src/components/resize-handle.css

[data-component="resize-handle"] {
  position: absolute;
  z-index: 10;

  &::after {
    content: "";
    position: absolute;
    opacity: 0;
    transition: opacity 0.15s ease-in-out;
  }

  &:hover::after,
  &:active::after {
    opacity: 1;
  }

  &[data-direction="horizontal"] {
    inset-block: 0;
    inset-inline-end: 0;
    width: 8px;
    transform: translateX(50%);
    cursor: ew-resize;

    &::after {
      width: 3px;
      inset-block: 0;
      inset-inline-start: 50%;
      transform: translateX(-50%);
    }
  }

  &[data-direction="vertical"] {
    inset-inline: 0;
    inset-block-start: 0;
    height: 8px;
    transform: translateY(-50%);
    cursor: ns-resize;

    &::after {
      height: 3px;
      inset-inline: 0;
      inset-block-start: 50%;
      transform: translateY(-50%);
    }
  }
}