Not possible to navigate over UI buttons with keyboard only. Reproducible on 5.2, 5.3, 5.4;

1. What happened
Keyboard-only navigation over UI elements doesn't work.

2. How we can reproduce it using the example you attached
a) Run the project, press up/down - nothing happens.
b) Click on a button with a mouse so the button has focus on it. Now pressing up/down(arrow keys) moves keyboard focus through UI buttons.

Expected:
a) Run the project, press up/down - a focus should appear that can be moved over buttons without first clicking on a button with a mouse.

Repro steps:
1. open attached user's project;
2. try to press arrow keys for navigation through UI buttons. That doesn't work. First you have to select any buttons to focus on it, then pressing arrow keys navigation works;

Project note: buttons are on display 2