Here we have another different root problem. Bookmarks on the homescreen are basically apps running in an iFrame.

STR:

1) Go to Ev.me
2) Bookmark Twitter
3) Open Twitter while you're offline

This UI should be the same than a hosted apps, I am pretty sure. BUT the frame type in the url says "regular". I did it for that reason. Gecko should distinguish between, I will say, iFrames for apps and bookmarks and iframes opened from apps (this is the case what you are talking about thought)