The mobile preview for 5.0 works like this:

• we show a page in a viewport with the dimensions of an iPhone and an iPad (minus their typical browser chrome) and inside the smartphone and tablet chrome we've used for 4.5
• the view is shown in the page preview only and below the page bar, which contains the switch to select the type of preview.
• we do not show the mobile preview in an overlay on top of the Desktop preview in order to
  • better support small screen sizes (the view is scrollable)
  • keep the Action Bar still available, also when in tablet preview
  • be directly able to open a specific mobile preview