Library group bar draws a garbage selection rectangle only in Big Sur

Big Sur has radically changed NSOutlineView. But it looks like I can sneak by with only a simple change to the selection style -- but I have to compile in Xcode 11 on Catalina. The same code that works when compiled in Xcode 11 Catalina, is totally broken and crazy when compiled in Xcode 12.

There are no errata or changes to the documentation about this totally new behavior.

I hate Big Sur so much.