It's quite difficult to produce a standalone-app that demostrates that bug.
However, there's a second issue that I haven't mentioned in my post and I could reproduce it with a demo-app.
Maybe the 2 are related to each other.
I've sent you the demo-app.
Attached is a short vid. showing what's wrong.
(A transparent window is created and left on the screen if there's a transparent panel on one of the created docked forms. Under Win7 the left-over form is black and therefore easier to see)
Ralf