To set the remote session to display on only one monitor when using the Windows Remote Desktop app, you'll need to adjust the settings before connecting. Since the Remote Desktop app doesn't allow changing display preferences from within the session, these settings must be configured on the local machine before you establish the connection.
Start by opening the Remote Desktop app. When setting up a connection, click on the settings icon or "Show Options" (depending on the version of the app you're using) in the connection setup window. Under the "Display" tab, you should see an option for choosing the number of monitors. Uncheck the option that enables multiple monitors, ensuring that only one monitor is selected. This should ensure that the remote session appears on only a single monitor during the connection.
If you're using a custom configuration or a saved profile, you may need to adjust this in the connection properties before launching the session. Once this change is made, you can save the configuration for future use, ensuring that the remote session always opens on a single monitor.
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hth
Marcin