The System UI Tuner was only introduced in Marshmallow, but it's already got quite a few features, with one graduating to a fully-fledged feature (and others causing quite a bit of controversy). The next thing Google seems to be working on for it is a navigation bar customizer, allowing quite extreme customization of the now standard back-home-recents navbar.

The navbar customizer is currently hidden in Nougat's code. Google's first commit containing the code was on January 22nd - it then removed the feature on February 24th because it 'wasn't ready' according to the GitHub commit message. A tipster has managed to re-enable the customizer by modding SystemUI, and has sent us screenshots and a description of each navbar modifier for good measure.

According to him, all the standard buttons on the navbar are removable, and new ones can be added at will. The new items include a clipboard button, a button whereby any keyboard function can be assigned to it, a spacer, and a menu / keyboard switcher. On the right-hand side of the screen, you will see a double-headed arrow, a cross, and two lines. These control and move the buttons; the arrow resizes the icon, the cross removes the button entirely, and the two lines allows for moving the item around on the navbar (i.e. right or left). Apparently the clipboard and keycode options don't work, so that's probably at least one reason why it was left hidden in 7.0 Nougat.

Maybe this will be turned on again for the maintenance release in the fall? Or maybe we'll be waiting for Android O to see it properly. Is it possible this code is what Google is using to create the Google Assistant-powered home button on the new Nexuses? Lots of questions and very few answers here.

UPDATE: 2016/08/24 10:50am PDT BY

Our tipster also posted a flashable ZIP on XDA-Developers for those who want to try it on a Nexus 6P. Note that this ZIP is only for the Nexus 6P - it won't work on any other phone running Nougat and may cause bootloops or other more serious issues. As usual, you're going to need a custom recovery (like TWRP) to flash the ZIP. You should also make a backup first and back out if you don't know what you're doing.

Thanks: Abdulwahab Isam, Andrzej Ressel