Yes, I believe the “process” looks something like this (failing to provide exact “2x” in step 3):
- Take the “resolution” (width/height) of the artboard
- For Retina displays (pixel density > 1x?) => Scale it up to match that pixel density (since XD is vector based and UIs, as well, can be scaled up, that shouldn’t be a problem, after all)
- Check if the resulting physical resolution (here: 3840*2160px) fits the screen. If it doesn’t, scale it down until it fits the screen, keeping the “2x” as “base resolution” that gets scaled down
Looking at your video, I could imagine that the screen’s width doesn’t even allow for the full 3840px (if the result is already 3492px), but even if it did, the height would be limited by: menu bar at the top, window controls, and the dock at the bottom => if the screen would allow for exactly 2x, that still wouldn’t allow the preview to be that big (unless it’s full screen), an therefore, it might scale down…
(Scaling down to fit the window, after all, also happens when you just resize the preview window)
All in all, I would assume that it tries to provide 2x, but has to scale down a little bit so that the window can fit your screen.