The Old New Thing

Why doesn't the window manager just take over behavior that used to be within the application's purview?

A commenter named "Al" wondered why the window manager couldn't just take over behavior that used to be within the application's purview, such as painting the non-client area, in order to avoid problems with applications not responding to messages promptly enough. If the window manager were being rewritten, then perhaps it could. But to do it...