Why can't I set width of artboard to 100% in prototyping?

Hi!

I perform external user testing regularly, but I am having problems with the XD prototypes. I updated to the latest XD Update today (18.09.2018) but this update still does not solve my problem.

Let's say I have an artboard that is 1280 px wide and 5000 px high

I need this prototype to be "full width" on a 1280 px wide screen, with the fold cut off at my viewport height like any other web page, and then scrollable with the browser scroll bar.

On a screen that is 1920 px wide, the prototype will then only take up 1280 px width, but still be scrollable.

1. I cannot set a viewport height, as I cannot know the viewport (device) of a user.
2. If I do know their device (and viewport height), and set a viewport height, I do not get the "browser" scroll-bar on the right hand side, so users cannot see that there is a way to scroll further down on the artboard. The scroll bar is invisible. The XD prototype is therefore useless for external user testing, unfortunately.

Thank you in advance.