Thanks for helping, Nick.
Your idea with the focus rectangle helped me to experiment.
I now think the problem is the viewport widget. If it has focus, player navigation with cursor keys does not work. If no widget (including the viewport widget) has focus, cursor keys work as expected.
If viewport widget has focus, clicking once into the game window has the desired effect of ‘unfocussing’ all widgets (including viewport widget).
2 pics for clarification:
one click later…
