User blog comment:Kirkburn/Technical Update: July 1, 2015/@comment-4189499-20150703141222/@comment-126761-20150703151038

We looked at a few options for this, and centered seemed to look the least odd. Dialogs that resize themselves can be difficult, and prone to problems - you wouldn't want important buttons suddenly disappearing off screen, for example.

We know it may seem a bit strange now, but I think it may be one of those things that rapidly becomes 'normal'. That being said, some kind of shading to indicate borders is an interesting thought - I'll pass it on.