From the currently available information, it seems like the only way to get the new-in-Tahoe sidebar inspector effect is to use a NSSplitView in conjunction with NSSplitviewController & inspectorWithViewController:.
I'm currently trying to get the same inspector effect - which also affects the looks of controls inside the inspector, like text fields, which switch to a gray-ish background - in a totally custom splitter-like view hierarchy that is way more complex than NSSplitView and thus cannot inherit or take advantage of it.
Is there a way to integrate this effect in a custom view? Maybe using NSVisualEffectView or NSGlassEffectView?