@Override
	protected void paintTabBorder( Graphics g, int tabPlacement, int tabIndex, int x, int y, int w, int h,
		boolean isSelected )
	{
		// paint tab separators
		if( clientPropertyBoolean( _tabPane, TABBED_PANE_SHOW_TAB_SEPARATORS, showTabSeparators ) &&
			!isLastInRun( tabIndex ) )
		  paintTabSeparator( g, tabPlacement, x, y, w, h );

		if( isSelected )
			paintTabSelection( g, tabPlacement, x, y, w, h );
	}
The content border insets are used to create a separator between tabs and content.
If client property JTabbedPane.hasFullBorder is true, then the content border insets
are also used for the border.
flatlaf-jide-oss/src/main/java/com/formdev/flatlaf/jideoss/ui/FlatJideTabbedPaneUI.java