I cannot test myself, but it seems that people using my app have only black window or something similar when the app opens an additional window. i would be grateful if people with macs could test if Screen works for them.
if yes, then the problem maybe with Viewport (or the two combined).
If it turns out to be an issue indeed with gideros and not with my code, then consider this as a bug report and a kind request to correct it (this i aim partially to @hg29
, the creator of the Screen feature).