It seems that no one has a clue. I guess I have to ask Apple directly.

You can try it with any button - it is the same. It works wich the blue button in the upper section though. IconButton is just a view, nothing else.

.frame(maxWidth: .infinity, maxHeight: .infinity) Doesn't work either.

Thank you for your comment. But the button is not clickable. It appears at the top but not clickable :-(.