Thanks for posting an example. Currently if you display a portrait document full-screen on a landscape device, the document expands to the width of the device and you need to scroll to see the bottom half of the document. This is what most people want because if they wanted to see the whole document, they would be using portrait orientation.
I could add a new behavior to keep the entire page visible, but I'd need to add a global setting for that to retain the current behavior as the default. (In that case, the same idea would apply to landscape documents displayed on a portrait device.) My development goals are already set for the rest of the year, but I might be able to fit that into one of the next few updates.
If you need an immediate solution, you could rework your one-page documents to fit onto a landscape page (imagine just one column of the example you attached here) and that would give the desired result.