Share a small part of the screen

[louism]

Is there a way to share, while presenting, a small part of only one window ? 

There is an option to share only one window, and another one to share a small part of the screen. But is there a way to combine both ? 

Thank you ! 

[Frank_TB]

Share screen>advanced> Portion of screen

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[louism]

Thank you for your answer Frank. Unfortunately, it doesn't work like I would like it worked. When using this option, the "part of the screen" shared follows every window I have. I would like to lock this functionnality on one window only. 
Do you know if something exists do to that ? 
Thank you !