code: @Override
      protected void paintContentBorder(final Graphics g, final int tabPlacement, final int tabIndex) {
          final int w = tabPane.getWidth();
          final int h = tabPane.getHeight();
          final Insets tabAreaInsets = getTabAreaInsets(tabPlacement);
  
          final int x = 0;
          final int y = calculateTabAreaHeight(tabPlacement, runCount, maxTabHeight) + tabAreaInsets.bottom;
  
          g.setColor(_tabSelectedBackgroundColor);
          g.fillRect(x, y, w, h);
  
          g.setColor(_tabBorderColor);
  
          // top line, except below selected tab
          final Rectangle selectTabBounds = getTabBounds(tabPane, tabIndex);
          g.drawLine(x, y, selectTabBounds.x, y);
          g.drawLine(selectTabBounds.x + selectTabBounds.width, y, x + w, y);
      }
docs: Paints the border for the tab's content, ie. the area below the tabs