Hi, Buck -
The reason you see differences in the appearance of your application running on different operation systems (even very old ones) is because...
The app is running on different operating systems.
I don't believe Microsoft has ever guaranteed rendering fidelity (identical look and feel) between OS versions. Your app will likely look a little different again on Windows Server 2016 and on Windows Server 2019. Just wait until the Windows 11 user interface is unleashed... I mean, released!
Except for the visual content of DataWindows, PB uses Windows controls for the various graphical user interface elements (windows, buttons, menus, etc.), so visual differences you see between OS versions are the result of changes Microsoft has made internally within the OS and not because of changes made by Appeon.
Best regards, John