private Insets getContentBorderInsets0( int tabPlacement ) {
		boolean hasFullBorder = clientPropertyBoolean( _tabPane, TABBED_PANE_HAS_FULL_BORDER, this.hasFullBorder );
		int sh = scale( contentSeparatorHeight );
		Insets insets = hasFullBorder ? new Insets( sh, sh, sh, sh ) : new Insets( sh, 0, 0, 0 );

		Insets contentBorderInsets = new Insets( 0, 0, 0, 0 );
		rotateInsets( insets, contentBorderInsets, tabPlacement );

		return contentBorderInsets;
	}
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