app/styles/components/resize-handle.scss

[data-resize-handle] {
  position: relative;

  &[data-panel-group-direction='horizontal']:after {
    content: '';
    position: absolute;
    top: 0;
    bottom: 0;
    left: -6px;
    right: -5px;
    z-index: $zIndexMax;
  }

  &[data-panel-group-direction='vertical']:after {
    content: '';
    position: absolute;
    left: 0;
    right: 0;
    top: -5px;
    bottom: -6px;
    z-index: $zIndexMax;
  }

  &[data-resize-handle-state='hover']:after,
  &[data-resize-handle-state='drag']:after {
    background-color: #8882;
  }
}