>Hi Bruce,
>
>Thanks for the additional information which is very useful. I still cannot pinpoint the problem but tonight I was able to work out which scenario would cause the problem. I am still investigating.
>
>I also noticed that the offending (top level) form leaves a trail when it is moved around. Did you find that when you app had the similar problem. By this I mean that the form leaves a mirror trail of itself as it is moved around the desktop. Weird!

That sounds like a classic symptom of having a form.LockScreen set someplace.