This widget is a shorthand to add margins to the sides of a child widget (which may of course also be a VBox or a HBox, i.e. several widgets).
Unlike more complex constructs like nested VBox and HBox widgets with VSpacing and HSpacing at the sides, the margins of a MarginBox have lower layout priorities than the real content, so if screen space becomes scarce, the margins will be reduced first, and only if the margins are zero, the content will be reduced in size.
1.4.4