The HD channels properly stretch to the full width/height of my screen.
1.) This wasn't initially the case. The SD channels were initially displaying correctly. It seems like this changed in the last week or two. Software update, maybe?
2.) It's not a TV display issue. I've already experimented with all the various aspect ratio modes and stretch settings on the TV. And other streaming apps (like Netflix or Amazon Prime) display fine with my usual 16:9 setting.
3.) Don't tell me, "Watch the HD channels, then." Not all channels are in HD. And the Xfinity Stream app, quite frankly, is a pain to navigate. If I'm channel surfing, and I happen to be on an SD channel, I just want to watch the thing without having it seem like I'm peering at it through a window, or having to go through a couple of sub-menus/search functions to find the HD equivalent.