Thanks for tracking this down. I have looked at the code changes between beta 5513 and 5829 and there is one change, relating to window regions and the MDI theme code, which is likely causing this. However, the change is intentional and I shouldn't really change it back to how it was in 5513. It may be that MDX/DCX are accessing custom @window regions in a way that is affected by timing or windows messages. It's difficult to say without looking at their code. I am going to make a change to the order in which regions are set in the next beta to see if that helps.