Android started to support Multi-display from Android 8.0, The document described as below:
If an activity supports multi-window mode and is running on a device with multiple displays, users can move the activity from one display to another. https://developer.android.com/about/versions/oreo/android-8.0#mds
I've tried to start an activity withe the flag "Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_LAUNCH_ADJACENT", and "setLaunchDisplayId" with secondary display. The activity I started is shown on the secondary display, but how can I move the activity from the secondary display to my phone?
I don't have enough rep to comment, I don't have an answer for you but I've come across some good reading material for multiple display.
https://source.android.com/devices/tech/display/multi_display
This link here indicates that as of Android 10 (API 29) that users can move activities on a device that is running multiple displays.
https://developer.android.com/guide/topics/ui/foldables#multi-display