I'm not sure how i can tell the problem correcty. So i made a simple project to show my problem to you.
1 mdi and 2 mdi childs forms are inserted to project. Each forms are maximized windows. When i want to view a form i see different size of forms. To see my problem please open sample project and hold enter. If i use ctrl+tab combination to view other form, there is no problem. But Zorder event fails.