Thanks, good catch.
Key.Return is valid code; this is just a cosmetic syntax-highlighting bug. The editor is colouring Return as the standalone Return keyword even though, after Key., it is being used as a member name. I've fixed the highlighter so keyword-like member names are not styled as keywords after a dot: https://feedback.objo.dev/bug/481
For Key.Return versus Key.Enter: in the underlying Avalonia input API they are aliases for the same key value. In Objo today, key events are reported as Return, so Key.Return is the spelling to use. I'm going to remove Key.Enter as its just confusing: https://feedback.objo.dev/deprecation/480