Partial MDI rewrite

[Mike Hearn]

On Fri, 16 Jan 2004 15:19:43 -0800, Alexandre Julliard wrote:
> The critical section thing worries me, especially since you are doing
> things that may require sending messages in there. I think you should
> use the normal USER lock when locking is really necessary; note that
> anything that is only accessed from the window procedure doesn't need
> locking at all.

One thing I didn't understanding looking at the DC code was the USER lock.
Why does USER not use CRITICAL_SECTIONs like everywhere else, instead of
these syslevel things? Are there MSDN docs on this or is it a purely
internal thing?