I suspect that'd play absolute hell with either their UX concept or the underlying UI abstractions / implementations.
This would create a situation where an application has one window foreground above the foreground application while it is not the foreground application.
On the UI / implementation side: I don't think it's possible to have any windows of an app foregrounded past the foreground app's frontmost window if that app isn't itself foreground. They'd have to create a new category of window (and probably move a bunch of internal data structures around) to make that possible. I know, "it's just code," but it's code based on some very deep and old assumptions about the way the window manager works that probably have hard-to-predict consequences if violated.
On the UX side: having a window floating foreground when the top-of-desktop menubar says another app's name in the corner is going to trigger a "WAT" for a lot of users, and I think Apple is deferring to them.
This would create a situation where an application has one window foreground above the foreground application while it is not the foreground application.
On the UI / implementation side: I don't think it's possible to have any windows of an app foregrounded past the foreground app's frontmost window if that app isn't itself foreground. They'd have to create a new category of window (and probably move a bunch of internal data structures around) to make that possible. I know, "it's just code," but it's code based on some very deep and old assumptions about the way the window manager works that probably have hard-to-predict consequences if violated.
On the UX side: having a window floating foreground when the top-of-desktop menubar says another app's name in the corner is going to trigger a "WAT" for a lot of users, and I think Apple is deferring to them.