Apple's X11 listserver offering, lists.apple.com Mailing Lists
or the Apple support forums, Apple - Support - Discussions - X11 ...
might be more fruitful.

I recall some folks installing a non-supported patch to X11 and/or reverting to a previous version of X11 than is supplied with Leopard.

There appear to have been bugs reintroduced and some feature "enhancements".

It's not specifically Unix that is having the problem you describe, but appears to be specifically X11 as supplied in the Leopard product.