Thank you very much GoToLoop for the hint into the direction of Processing.js and thereby to Javascript. Infact it showed that ‘window.innerWidth’ and ‘window.innerHeight’ are functional replacements for ‘displayWidth’ and ‘displayHeight’. It works fine now!!
Btw, only later I had realised that the Processing App that I had found in the App Store Germany comes from a third party. Might be that the App Store in other countries offer different Processing Apps though.