If you're app is still targeting Windows 8, then you still want to use the explicit view states. For Windows 8 apps, Windows 8.1 allows variable resizing of the view, but the app itself will still see only snapped, filled, and fullscreen states. The letterboxing
you see is by design, because Win8 apps don't expect to be in other states between snapped and filled.
To do true variable sizing using min-width/max-width, you need to retarget the project for Windows 8.1. If you want best compatibility with both OS versions, at present you'll need to maintain a separate build for each.
