Using Dell P2419H Monitor with MacBook Pro (Retina, 13-inch, Early 2015)

I just bought a Dell P2419H monitor to use with my 2015 MacBook Pro, connecting it using the HDMI port.

I can't figure out why the picture is underscanning on the monitor. i.e. there is a think black border around the picture.

I've tried two separate HDMI cables, playing with the Display settings, researching online, etc. all to no avail.

Does anyone have any clue how I can fix this? Or is the monitor simply not compatible with my laptop?