Although I like this idea, I wouldn't want this feature to get prioritized over making the layout "responsive" on mobile browsers.
(I've tried a few HN "apps" on both iOS and Android. Either they reinvent the UX too much for my taste, or they're no-login/read-only ... or even if they get those points correct, they have intermittent "no comments" errors when there are in comments. Sigh. Really I just want the web site. Albeit updated to work better on smaller screens.)