@broberg Well, I appreciate your advice it was very helpful. It was my intention to separate the RPi from the display and build a small mirror using a relatively cheap tablet. After editing the config.js file all works fine now with a fullscreen browser app. Besides using a tablet makes it easy to use the front camera as a motion detector to switch the mirror. Thanks again!
MagicMirror² v2.20.0 is available! For more information about this release, check out this topic.