How can I prevent apps from going full screen and hiding the notification bar? The notification bar takes only a few pixels yet contains so much useful information. Unfortunately, there are some apps (that I otherwise love) that insist on being in full screen, and therefore remove the notification bar from sight.

Is there any way I can force the notification bar to remain while running these apps?

I run rooted Marshmallow with Xposed Framework on a Samsung SM-G900F.

The apps can run on all types of display sizes and aspect ratios, so technically they should be able to do without those pixels on top.
There is an app for your required feature Status bar in full screen

You can use it with any launcher and it is compatible with Android 2.2+

In the app, you can toggle status bar on/off in fullscreen mode.