@Override
	protected Insets getContentBorderInsets( int tabPlacement ) {
		return FlatUIUtils.addInsets( getContentBorderInsets0( tabPlacement ),
			scale( super.getContentBorderInsets( tabPlacement ) ) );
	}
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