How can I disable multiwindow mode for an Activity in Android N+
In your manifest, you need:
android:resizeableActivity="false"
So in your manifest file, for each activity that you want to disable the feature in, it would be like:
<activity android:name=".SomeActivity"
android:label="@string/app_name"
android:resizeableActivity="false" />
Or, if you want to disable it in your entire app:
<application
android:resizeableActivity="false" >
. . .
</application>
As for what will happen, Android just won't let your app go into multi-screen mode - it will just stay full screen. See https://developer.android.com/preview/features/multi-window.html and https://developer.android.com/guide/topics/manifest/activity-element.html#resizeableActivity.