I have connected a 23" LCD TFT via Micro-HDMI.
If the LCD is connected before Power-On the Bios, Boot and Ubuntu-Output are only shown own the LCD and not on the internal display.
Even if i use to change the settings via xrandr, the internal display does not show anything although the configuration seems to be ok.
If the LCD is not connected before Power-On, everything is shown on the interal display.
As soon as (x)ubuntu is started i connect the LCD TFT via HDMI and use xrandr to setup the configuration, everything is ok, the desktop gets extended just as usual.
xrandr --output HDMI1 --mode 1920x1080 --pos 0x0 --rotate normal --output DP1 --off --output eDP1 --mode 1920x1080 --pos 1920x0 --rotate normal --output VGA1 --off