some apps like document viewer can't get fullscreen on tablet

[Rain]

On the teblet,in order to show two apps at a time, some apps are designed to take over only half of the screen. These apps, for examples, are system settings, document viewer, twitter, droping letters, etc. But the above apps can't be turned into fullscreen, and their size can't change, either. This can be very annoying when I read PDF documents because the window is too small.
 To solve this problem, at least, the halfscreen app in landscape should turn into fullscreen when portrait. When this is done, make the half screen apps resizeable, which could turn into fullscreen by a gesture(or a button, etc) both in landscape and portrait. At last, make every app resizeable and can get full/halfscreen by a gesture（just like the desktop).