Problem:

The Opacity allows to get some translucent effect on Windows so that the underlying desktop or windows can be seen.

If however the property set on a MDI Clhild Window, it is possible to get unexpected painting problems in the application.

Resolution:

This is a restriction imposed by Microsoft.

Windows and WinForms do not support MDI Child Windows having opacity set.

Setting this in the application on MDI the application may have issues and unexpected behaviour when Windows are painted.