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I like to read on my phone. Sometimes an ebook while in a waiting room, sometimes Reddit. I use a ZTE Axon 7 and not an LG or other OEM with a "smart stay" feature that looks to see if you're looking at the screen. How do I keep the screen on when I'm actively using it but not touching the screen? KinScreen! KinScreen - Android Apps on Google Play.
What I do is set the angle in "Keep the screen on when tilted up more than" at 20 degrees. This means that if my phone is angled 20 degrees or more, the screen stays on. If it's flat on a surface, it goes off. My screen never goes off while I'm holding it and reading something. A separate setting is to turn the screen off if the proximity sensor is covered (in my pocket) for 10 seconds. These settings can all be customized to the user's liking. As each sensor is triggered, it'll turn green up top.
Edit: I've also disabled this app's ability to show notifications, as it will show a persistent notification. It's not necessary for the app to function.