_input() not working when node is child of a viewport
If I place the node on the top hierarchy it works fine, but when I put it under a viewport, _input() stops working entirely.
I even have a print statement in there, and nothing gets printed when under the viewport.
I found someone else with the same problem: https://godotengine.org/qa/9751/_input-event-doesnt-work-as-child-of-viewport
but I'm not figuring out how the proposed solution is supposed to be implemented (and the OP didn't give any feedback).
I also would like to know why this happens.