As the subject title suggests, I have included a couple of screenshots that show the problem which significantly reduced the functionality of some UWP fluent apps (have not seen this with Win32, yet).
This shows the Windows Mail/Calendar app with and without a curtains theme applied and you can see the back button which sits on the title bar is not accessible. In this case, the user simply cannot us the pop out feature at all, to return to the main windows without closing the app fully and restarting.

Without the theme as normal, below ...
