As usual: after I searched for a topic without success, I post in a forum, search again .... and find the solution. On stackoverflow there is a post describing this problem and the answer for NavigationView https://stackoverflow.com/questions/67327453/geometryreader-with-navigationview-in-swiftui-is-initially-giving-zero-for-size). The solution is to use ".onChange(of: geo.size)".

The bug is still there with iOS 18.1. I wrote an app on iOS 17 and updated my iPhone yesterday - to discover this problem...