How to enable MultiWindow on Android Marshmallow!

Boooom! Android M preview is out. For those that already flashed it, there might be a really really cool thing for you to try. In fact, Android M comes with multiwindow. It's experimental but seems to be working. Let's try it out!

1. Firstly, boot into recovery. Be sure to have a custom recovery, TWRP works the best.
2. Be sure the /system partition is mounted (Go to mount -> mount the system)
3. Connect your device to your PC, be sure to have your device recognizable by ADB, type adb devices - your device should pop up. (Don't have ADB? Download here!)
4. Let's access the build.prop, type the following commands in to the terminal:

adb shell

cd /system

vi build.prop

Let's change a line to enable userdebug mode. Change the line:

ro.build.type=user

to

ro.build.type=userdebug

Reboot your phone. Navigate to developer options in settings (to enable dev options, go to "about phone" and tap on "build number" several times).

Open recents and boom, you have it.

Source