Comment by asimpson

You can achieve something similar using vlc + slop on Linux, here's the script I usually use (I run i3 typically).

    selection=$(slop -f %w,%h,%x,%y)
    width=$(echo "${selection}" | cut -d , -f 1)
    height=$(echo "${selection}" | cut -d , -f 2)
    top=$(echo "${selection}" | cut -d , -f 4)
    left=$(echo "${selection}" | cut -d , -f 3)

    cvlc --no-video-deco --no-embedded-video --screen-fps=20 --screen-top=$top --screen-left=$left --screen-width=$width --screen-height=$height screen:// &

Then "just" share the VLC window instead of your desktop.