public class Separator

A rule that divides one group of controls from the next.

Every desktop toolkit draws one – NSBox in separator mode, WinUI’s MenuFlyoutSeparator and navigation separators, GtkSeparator – and Codename One had no component for it, so applications drew their own out of a Label with a bottom border, or a Container with a fixed height and a background colour. Those look approximately right on one platform and wrong everywhere else, because the thing that differs between platforms is exactly the part they hard-code: the thickness, the colour, and how much air sits either side of it.

All three come from the Separator UIID. The rule itself is the style’s border where there is one and its background colour otherwise, the air either side is the style’s margin, and the thickness is the separatorThicknessMM theme constant. A theme that says nothing gets a one-pixel line in the foreground colour.

Container settings = new Container(BoxLayout.y());
settings.add(new Label("Appearance"))
        .add(new Separator())
        .add(new Label("Privacy"));

A separator is not focusable and is skipped by keyboard traversal: it is decoration, and stopping on it with Tab would be a bug on every platform.