U3415W, no full screen, check the GPU scaling setting

This could be a limitation of the laptop. Did you test both bottom HDMI in ports on the U3415W? Does the laptop have DP or mDP out ports to test on the DP/mDP in ports on the U3415W? Have you tested the U3415W and that HDMI cable on a desktop computer to see if it has the same issue?

Below is the testing of my two U3415W using HDMI out ports on my laptops =
U3415W MHL HDMI 1.4 (Latitude E7240 3440x1440 50Hz)
U3415W HDMI 2.0 (Alienware 17-R3 3440x1440 60Hz, Latitude E7240 2560x1080 59Hz)

I don't know why people can't just give you a straight answer. I had the same problem and called tech support from the store I purchased the unit. It was a lot more complicated than I would have guessed (not that I'm a tech savvy person anyway--I have my days).

I'll just give you the example from my laptop. I purchased the monitor along with a Dell laptop (xps 15 9560 Signature Edition).

With both the monitor open and your laptop, right-click on the laptop monitor (anywhere).

Select: Intel Graphics Settings (this will take you to the Intel Graphics setting control panel)

In this panel you will have several options on the display, including aspect ratio, scale to full screen, etc..

I currently have mine set to Scale Full Screen, maintain aspect ratio, with a refresh rate of 60Hz. The resolution is set to maximum at 3840x2160. I'm also using, Built-in display.

In the Intel Graphics Settings, you may have to play around with the settings to get things to where they need, but these settings will assuredly get you the results you are looking for. Good luck and I hope this helps;)

So the black bars were an indication that the aspect scaling was set incorrectly in the Intel Graphics control panel. Good catch!