Currently, when you test a prototype on a mobile device it will fit the prototype to the width of the testing device, despite the fact the zoom mode is called “Fit to screen” or “contain” in the URL request param. Which results in a bad experience when the prototype is designed in a higher aspect ratio than the device you’re testing the prototype with. Main CTAs placed on the bottom of the screen will appear underneath the fold for example.
It would be ideal if you select “Fit to screen” the artboards will actually fit the screen. So if the testing device has a less high aspect ratio than the artboard, space will be added to the sides of the prototype. And if the testing device has a higher aspect ratio than the artboard, space will be added to the top and bottom of the prototype.
The “Fit width” zoom mode can be used for the current behavior.