UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.84 Safari/537.36

Steps to reproduce the problem:
1. Attach and configure multiple monitors on a Linux desktop
2. Generate media stream with chrome.desktopCapture.chooseDesktopMedia(...)

What is the expected behavior?
The client should be able to choose one of the screens, windows, etc. and obtain the stream of *just* the specified content.

What went wrong?
Regardless of the user's choice of what to capture (in my testing even choosing a single window to capture is ignored), all monitors are streamed simultaneously as one larger screen.

In the attached screenshot, the smaller section on the left (2, outlined in red) is a secondary monitor; the section on the right )1, outlined in blue) is the main monitor. This screenshot was taken while screen sharing on Discord, which is one of the applications affected by this bug.

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 68.0.3440.84  Channel: stable
OS Version: Debian 9.5 (Stretch)
Flash Version: Shockwave Flash 30.0 r0

This issue encompasses a multitude of other issues that arise from this bug, given that the chrome.desktopCapture API is widely used. The bug seems to have persisted for almost almost 4 years now, without any action to fix it.

The bug was previously discussed in
https://bugs.chromium.org/p/chromium/issues/detail?id=660032
which was marked as a feature request. It discusses the bug in more depth, and includes some testing of affected applications.

Chrome on Windows and OSX properly handles multi-monitor setups. There is no reason to believe support for this feature on Linux is impossible; see https://bugs.chromium.org/p/chromium/issues/detail?id=660032#c11:
>   ...this is a bug, not a feature; feature-parity is missing in this regard -- X11 proto/spec supports this...

Further discussion/references, organized by where the bug manifested (listed roughly in order of usefulness):
---
Google Hangouts
    https://bugs.chromium.org/p/chromium/issues/detail?id=660032 [Linked above]
    https://bugs.chromium.org/p/chromium/issues/detail?id=437507
    https://unix.stackexchange.com/questions/152435/sharing-your-desktop-with-google-hangouts-dual-monitor-and-gnome-shell
    https://productforums.google.com/forum/#!msg/hangouts/JKc_v0lMTTg/gk7Rd5cZAgAJ

Discord
    https://feedback.discordapp.com/forums/326712-discord-dream-land/suggestions/19416319-linux-multihead-screen-sharing-caveats-on-electron
    https://www.reddit.com/r/discordapp/comments/8540z0/how_discord_handles_duel_screen_linux/

Jitsi-Meet
    https://github.com/jitsi/jitsi-meet/issues/2894
 

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