public void setTabUnderlineHeight( int tabUnderlineHeight ) {
		putClientProperty( TAB_BUTTON_UNDERLINE_HEIGHT, (tabUnderlineHeight >= 0) ? tabUnderlineHeight : null );
	}
Specifies thickness of underline if toggle button type is {@link ButtonType#tab}.
flatlaf-extras/src/main/java/com/formdev/flatlaf/extras/components/FlatToggleButton.java