Thanks for the tip. Now knowing where to look, I inspected the following key on the Windows 10 machine.
HKEY_CLASSES_ROOT\Applications\photoviewer.dll\shell
It contained the "print" sub-key but...

System File Checker? I've already tried the "nuclear option" which effectively reinstalls the operating system:
Settings > Update & Security > Reset this PC


Am I correct in assuming that...

Latest relevant update was installed July 18th:
Security Update for Windows 10 for x64-based Systems (KB3074665)

Right there in my first bullet point: "Windows Photo Viewer is never listed as an option when using "Open with".



That would work if Windows Photo Viewer were in the list but it is not.

For...

Thanks; I've been using Faststone Image Viewer for several years. However, that's not what I want as my default image viewer when I simply want a quick look at a photo or three. Windows Photo Viewer...

I'm not making any headway on answers.Microsoft.com so I'm hoping the experts here can explain why I cannot replace the Photos app with Windows Photo Viewer as the default image viewer (in build...