@Override
	protected void paintTabArea( Graphics g, int tabPlacement, int selectedIndex ) {
		// need to set rendering hints here too because this method is also invoked
		// from BasicTabbedPaneUI.ScrollableTabPanel.paintComponent()
		Object[] oldHints = FlatUIUtils.setRenderingHints( g );

		super.paintTabArea( g, tabPlacement, selectedIndex );

		FlatUIUtils.resetRenderingHints( g, oldHints );
	}
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-core/src/main/java/com/formdev/flatlaf/ui/FlatTabbedPaneUI.java