code: public void paintSplitPaneDividerBackground(SynthContext context, Graphics g, int x, int y, int w, int h, int orientation) {
          if (orientation == JSplitPane.HORIZONTAL_SPLIT) {
              AffineTransform transform = new AffineTransform();
  
              transform.scale(-1, 1);
              transform.rotate(Math.toRadians(90));
              paintBackground(context, g, y, x, h, w, transform);
          } else {
              paintBackground(context, g, x, y, w, h, null);
          }
      }
docs: Paints the background of the divider of a split pane. This implementation
  invokes the method of the same name without the orientation.
  
  @param context     SynthContext identifying the <code>JComponent</code>
  and <code>Region</code> to paint to
  @param g           <code>Graphics</code> to paint to
  @param x           X coordinate of the area to paint to
  @param y           Y coordinate of the area to paint to
  @param w           Width of the area to paint to
  @param h           Height of the area to paint to
  @param orientation One of <code>JSplitPane.HORIZONTAL_SPLIT</code> or
  <code>JSplitPane.VERTICAL_SPLIT</code>