[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.

[dctoedt]

Brings to mind the infamous comment about Dropbox: https://news.ycombinator.com/item?id=9224

EDIT: As noted below by @cole-k, the situations aren't the same, because the parent comment here is talking about approximating DeskPad-like functionality on a platform not (currently) supported by DeskPad.

[cole-k]

I feel this is a little unfair to the parent as they are offering a solution for an unsupported platform.

[dctoedt]

Point taken; I edited my comment.