Select a monitor
Start a screen recording and use the browser or app capture chooser to select the display you need. Check its preview before confirming. A window or tab capture follows a different source from a whole-monitor capture.
Need content from more than one display
Move the relevant windows onto the selected display before recording, or record separate sessions and keep their files clearly named.
Do not rely on opening a second tab to guarantee simultaneous capture of another monitor. Capture sessions and permissions depend on the host app, browser and operating system. Test the exact arrangement before using it for a session you cannot repeat.
ScreenApp does not automatically turn separately recorded screens into one synchronized video. Check each saved recording for picture and sound.
See browser recording and Windows recording for the capture steps.