The UX tests we ran prior to 4.5 release suggested, among other things, some "minor" improvements to the new page editor. One of them was having the main bar (a.k.a. page bar) always on top when you scroll the page up and down in authoring mode. This has a couple of advantages:

• It acts as an indicator that you're editing (if not many bars are visible, that's sometimes less obvious than one thinks)
• people using the mobile preview a lot would benefit from it (not if you mostly use the desktop preview, because in that case you have the handy ESC key shortcut to switch between edit and preview modes), as do customers that extend the bar to e.g. show the activation change of a page - which seems to be done frequently.

We asked Magnolia's users to have their say before shipping this feature, see blog post here http://dev.magnolia-cms.com/blog/2012/05/page-editor-main-bar-fixed-on-top-cast-your-vote/