AndroidPresentationDisplayDemo

Demo app for the Android Presentation Display component.

How to Grab a Screenshot from the Secondary Display

You can get the "Display ID" using:

adb shell dumpsys display

And then look for a display labeled like "HDMI Screen". The Display ID is the numeric value of the uniqueId property in most cases:

DisplayDeviceInfo{"HDMI Screen": uniqueId="local:1", 1280 x 800, modeId 2, 
defaultModeId 2, supportedModes [{id=2, width=1280, height=800, fps=60.000004}], 
colorMode 0, supportedColorModes [0], HdrCapabilities android.view.Display$HdrCapabilities@40f16308, 
density 237, 237.0 x 237.0 dpi, appVsyncOff 1000000, presDeadline 16666666, 
touch EXTERNAL, rotation 0, type HDMI, address {port=1}, state ON, 
FLAG_SECURE, FLAG_SUPPORTS_PROTECTED_BUFFERS, FLAG_PRESENTATION}

Then you use the -d option in adb shell screencap:

adb shell screencap -d 1 /sdcard/screen-01.png && adb pull /sdcard/screen-01.png ~/Desktop/

The same concept should apply to screenrecord, but this is only available on Android 11+. Android developer documentation:
https://developer.android.com/tools/adb#screencap

adb shell screenrecord --display-id 1 /sdcard/recording-01.mp4
adb pull /sdcard/recording-01.mp4 ~/Desktop/

Stack Overflow post:
https://stackoverflow.com/questions/64180082/adb-screenrecord-secondary-display-from-listed-displays-in-dumpsys

Link to screenrecord source code:
https://android.googlesource.com/platform/frameworks/av/+/refs/heads/master/cmds/screenrecord/screenrecord.cpp

How to screencast with Genymobile screen copy

scrcpy also supports a --display flag:

scrcpy --display 1

You can also record the secondary HDMI screen using scrcpy like:

scrcpy --display 1 --record ~/Desktop/recording.mp4

More details:
https://manpages.ubuntu.com/manpages/jammy/man1/scrcpy.1.html
https://github.com/Genymobile/scrcpy