tl:dr for anyone needing help. Make sure the “Force Module Active” is checked on the scene’s EventSystem.
Starting trials and errors:
Plugging in the Xbone controller while in the Unity editor shows Joystick connected ("Controller (Xbox One For Windows)").
which is expected.
In Project Settings → Input, Horizontal and Vertical are both using “Joystick Axis” “X/Y Axis” (depending), and “Get Motion from all Joysticks”.
Input.GetJoystickNames() returns “Controller (Xbox One For Windows)” in Unity Preview Preview.
Input.GetJoystickNames() returns “Controller (Xbox One For Windows)” in Standalone. I checked the Input window for Standalone, and noticed that it defaults to “joystick 0 axis 0” for horizontal, and “joystick 0 axis 1”, and after changing the input (it detected “Joystick 1 axis 0” and “Joystick 1 axis 1” for horizontal and vertical, respectively) it works.
Input.GetJoystickNames() returns “Xbox 360 Controller for Windows” in UWP Windows 10, which is wrong. Just noticed, also, that even though the screen is auto-focused after launching from VS2015, I still have to click the window, or at very least alt-tab to it again, to get it to use the Xbone controller. This is better than it was before.
Input.GetJoystickNames() returns “Xbox 360 Controller for Windows” in UWP Xboe One, as well. After adding a spot for GetKey(“Y”) with some on-screen text, it is detecting key presses, so the controller is working.
Digging down deeper, I noticed I had added a second “Standalone Input Module” to specify A, B, JoystickX, JoystickY for the appropriate fields, and that Vertical’s default is Inverted and JoystickY’s was not, so even if it was working, they would be cancelling each other out. I removed the second Standalone Input Module and stopped referencing JoystickX/Y. I can now see values for Horizontal and Vertical, but the UI still isn’t responding.
So current situation: controller is working, I just hadn’t tested it enough. However, it’s reading as an Xbox 360 controller on the Xbox One (odd) and the UI is not responding in any way, shape, or form to input.
Final solution: DON’T add a second “Standalone Input Module” to the EventSystem (though verdict is out on if this caused any issues and I don’t feel like messing around with this right now to check), but make sure the “Force Module Active” is checked on the one that comes default. This looks like it was the main culprit, and why it felt like the controller wasn’t connected. There must be some weird bug where the EventSystem starts before the game has detected a controller, and so that script just never gets initialized correctly or something /shrug.
Sorry for the train of thought work-through, here. Hopefully people wanting to get a controller set up and working with Xbox One UWP Dev will see this first instead of making the same jumps in logic that I did.
Thank you, guavaman, for jogging my quick-and-dirty-debug memory to print things on the screen. I was contemplating just grabbing Rewired to get around this, but can’t rationalize spending the money until I’ve made a few more sample games.