The old "it works on my machine" bleat. Past age twelve that is not an acceptable response when someone points out a problem. The acceptable response is "how can I reproduce that so I can fix it?"On my iPhone, SB goes from screen edge to screen edge, no "whitespace".
An iPhone is not an iPad. Apple made quite a big point of that when explaining to developers that a simple port of an iPhone app to the iPad was missing the point.
One size does not fit all.
While we are talking about iPhones (6s), the new post screen has so much extraneous rubbish at the top that the title of the first (ie most recent) post in the lists of posts is only just (and not completely) visible in portrait mode.
So you don't have to scroll sideways but you have a lot of up down scrolling to do.