It this intended behavior? I guess it could be if that is standard in macOS - on the other hand it is a bi confusing in bug controls with a ton of data columns that are no splitter/border/separator/
I'd say it's missed implementation. Yet I'd think there should be a basic implementation in TWidgetSet itself.
I don't really see, how drawing a rectangle edges (with Windows defined styles) could be any OS specific.
The system could supply colors (which is critical for dark theme for sure), but other than that, I can't think of anything else Cocoa could be useful for.