Multi-Window coming to stock Android?
Multi-window is referenced in AOSP Code Commit
The multi-window technology available on the Samsung Galaxy Note phablets might soon be available to many more Android users. The day after Google officially unleashed Android 5.1, an AOSP framework Code Commit was found that has a reference to multi-window. The feature allows users to divide their display into two halves with each one running a different app.
The brief mention of multi-window really doesn't give us a clue as to when such an app would be made available, or how it would work on a non-Galaxy Note Android phone. Back during the first month of 2014, a concept video showed how apps like WhatsApp, Snapchat, Twitter and YouTube could be used on Android within a multi-window application.
Google I/O 2015 will be held May 28th and May 29th. There is a possibility that we could hear something more about this during the annual developer conference. At last year's Google I/O, Android "L" was unveiled. This, of course, became the Android Lollipop 5.0 build.
source: Google via Phandroid