There are some smartphone features that have become so standard that many of us automatically assume every new phone will have them. Double tap to sleep/wake is one that I assume a smartphone with an AMOLED screen will have and it seems like we're getting to that point with the always-on display as well. The OnePlus 6 had the always on display feature removed due to battery concerns, but there's a Magisk Module that will bring the feature to the OnePlus 5, OnePlus 5T, and the OnePlus 6.
How to enable Always on Display on the OnePlus 6 & OnePlus 5/5T
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