How to mirror a selected area/window in primary display to second display
OBS — Open Broadcaster Software


In System Preferences → Displays, do not use Mirror Displays.
In OBS, click the + button under Sources and choose Display Capture.


Set ‘Display’ to your normal display.
Set Crop to your preferred method of cropping, such as Manual or To Window and configure the crop settings below. Click OK.


Use the red handles to resize your Display Capture source appropriately within the black ‘display’ (preview) in the OBS window. This is how/where your display will show on the mirror.


Right-click anywhere in the preview and choose ‘Fullscreen Projector (Preview)’, then select your external display to output to.