Thanks for the test scripts. It looks like the issue is indeed related to windows regions. MDX appears to be creating/redirecting/hooking windows/messages in order to capture the output of the /draw commands. This results in mIRC not receiving sizing messages for the window, so it cannot update the window region itself, and MDX is not setting the window region appropriately (most likely because, in previous versions, this was not needed).

The way beta 5829 handles this is technically correct. However, I am going to make a change that should resolve the issue with MDX (and possibly the DCX issue reported in another thread). This change should be in the next beta.