What do you expect? You make your UI thread busy with lengthy operation, why could you expect some UI activity? By miracle?
And this is not related to a form. Why do you think that forms work independently?
You need to execute all blocking or lengthy operations in a separate thread, other then UI, to keep UI responsive. The only problem is: you cannot call anything related to UI from non-UI thread. Instead, you need to use the method
(for both Forms or WPF) or
You will find detailed explanation of how it works and code samples in my past answers:
Control.Invoke() vs. Control.BeginInvoke()
Problem with Treeview Scanner And MD5
See also more references on threading:
How to get a keydown event to operate on a different thread in vb.net
Control events not firing after enable disable + multithreading