Immersive mode (fullscreen) paralyses user. And how to deal with it.
Back and Undo are the only safe points in every app
User hits undo/back button after mistake or getting back. He need not to think about it as about every other UI element. Even our grandmas know how to use back button.
I know you implemented immersive mode according to Android Guidelines, but this particular Google guideline suck because it takes the only certainty away from users.
When immersive mode was introduced (I had fullscreen before), it took me a minute to realize how to even get out of the app because there were no buttons at all!
How good apps implement Immersive mode
Good apps keep a little visual clue on place of buttons. Full symbols are shown on tap and their function works without change. It does not interfere with main purpose of the app because these buttons are located on border, but it allows all screen to be used for an app.
See wireframe: Immersive-done-well.png example of Camera app.
1) How Locus could implement immersive mode
Locus could follow this UX pattern by keeping those visual clues (dots) on place of buttons and keeping it's function/visualization on tap. It could even keep the menu button in top left/right.
Buttons with full symbols should be visible at start and fade into dots after a second two.
This would does not distract users while keeps the main functionality.
See wireframe: immersive.fw.png
2) How can Locus redesign the whole app to properly use a screen real estate
All the buttons could be solved as overlays with borders/shadow to be visible on all kind of backgrounds.
See wireframe: immersion-overlays.fw.png
Thanks for considering at least 1) I can create a followup suggestion for 2).
You can get inspiration also by mapy.cz app which do that really well.