Discovered the reason: the "instance" is actually only a reference in the code location shown. So after making it a real instance the code completion does work.
Before:
var children = List[Node[T]]
After:
var children = List[Node[T]]()
Now I did actually realize the reference had not yet been initialized: but had not considered that IJ needed that to happen before being able to display methods that ALL List's provide.
The screenshot below shows the changes : initialized the instance and popup subsequently behaving as expected.