Fool's design.Office 2013 - Mysterious invisible moving handles in Outlook.
You can move the Outlook window by grabbing the handles marked in green but not the section marked in red. To confuse the users even further, the red section has not a fixed size. If you resize the Outlook window, the red section resizes! Having an anchor in the right, there is almost no way to guess where the left border of the red section will be, hence moving the Outlook window becomes a guessing game.
Business as usual for Microsoft design team.