Samsung Galaxy Note 5 Manual page 60

Choose Apps That Are Already Running
When choosing which apps to start in Multi Window mode, if you keep scroll-
ing right so that you see the left-most screen, it looks very much like the Recent
Apps screen. Tap an app that is already running to switch it to Multi Window
mode.
Tap to choose
a running app.
It's Not All Good
Not All Apps Support Multi Window
Apps must be specially written to take advantage of Samsung's Multi Window
mode because Multi Window mode is not part of the Android operating sys-
tem. This means that you might not see the apps you are looking for until the
developer updates the app to support Samsung's Multi Window mode.
Fundamentals of Android 5.1.1 and TouchWiz
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