Separator
A 1px rule for the places where whitespace alone does not show that one group has ended — reach for Stack gaps first, because a drawn line is a statement. Most families ship their own (DropdownMenuSeparator, SelectSeparator, FieldSeparator, NavRailSeparator and the rest) already carrying that context’s defaults; the raw primitive is for generic content. It is decorative by default, so set decorative={false} only when the line marks a boundary a screen-reader user should hear.