That's not how Windows GUI works, to some extent it's the other way around with all controls being implemented in client's user space code. The difference is that for windows, there is one standard client toolkit that is part of the OS API. BTW, even window decorations are drawn by the client.

The kernel side component does even somewhat less than X server, as all drawing is done either by direct writing to frame buffer from userspace or thru DRI-like direct rendering mechanism. Kernel side "display server" then only handles message queues and input and tracks which screen region belongs to which window (and set's up the shared memory accordingly, in theory).

For 16 bit windows, one might say that the userspace libraries are somehow part of the kernel (as there is no user/kernel split in win16 and one of the libraries is even called kernel.dll), but in win32 case these libraries are userspace.

On the other hand, NeWS principle is that server implements turing-complete virtual machine (based on postscript) that executes arbitrary code uploaded from clients (which then can define behavior of widgets, specify new ones or even create windows whose complete behavior is server-side).