public ExpandableSide getExpandableSide() {
		return getClientPropertyEnumString( SPLIT_PANE_EXPANDABLE_SIDE, ExpandableSide.class,
			null, ExpandableSide.both );
	}
Returns what side of the spilt pane is allowed to expand
via one-touch expanding arrow buttons.

@since 3.4.1
flatlaf-extras/src/main/java/com/formdev/flatlaf/extras/components/FlatSplitPane.java