Overscan and HDMI Modes
Note: This note was originally in the Raspberry Pi 2/3: Chromium in Kiosk mode post.
In some cases the image may not fill the screen.
The overscan setting below can help, as well as forcing certain HDMI modes as document [here](From https://www.raspberrypi.org/documentation/configuration/config-txt/README.md) and here.
/boot/config.txt to change these settings, for example
is 1080p@60Hz when
disable_overscan=1 hdmi_group=1 hdmi_mode=16
The difference may be subtle here.