Sharing a single screen in Slack for Linux

I have a bunch of monitors, and for whatever reason, Slack for Linux refuses to let me limit my screen sharing to a single monitor or application.  This means that if I try to share my screen on a call, no one can see or read anything because they just see a giant, wide view of three monitors' worth of pixels crammed into their Slack window (typically only one monitor wide).

In my experience, disabling monitors/displays is just not worth it; I'll have to spend too much time getting everything set back up correctly afterward, and that's really inconvenient and annoying.

The solution that I've landed on is Xephyr; Xephyr runs a second X11 server inside a new window, so when I need to get on a call where I'll have to share my screen, I simply:

1. Launch a new Xephyr display.
2. Close Slack.
3. Open Slack on the Xephyr display.
4. Open whatever else I'll need to share in the Xephyr display, typically a web browser or a terminal.
5. Get on the Slack call and share my "screen".

Some small details:

• You'll need to open Xephyr with the resolution that you want; given window decorations and such, you may need to play around with this a bit.  Once you find out what works, put it in a script.
• In order to resize windows in Xephyr, it'll need to be running a window manager.  I struggled to get any "startx"-related things working, but I found that "twm" worked well enough for my purposes.
• Some applications, such as Chrome, won't open on two displays at the same time.  I just open a different browser in my Xephyr display (for example, I use "google-chrome" normally and "chromium-browser" in Xephyr), but you can also run Chrome using a different profile directory and it'll run in the other display.

Install Xephyr and TWM:

sudo apt install xserver-xephyr twm

Run Zephyr, Slack, and Chromium:

# Launch Xephyr and create display ":1".
Xephyr -ac -noreset -screen 1920x1000 :1 &
# Start a window manager in Xephyr.
DISPLAY=:1 twm &>/dev/null &
# Open Slack in Xephyr.
DISPLAY=:1 slack &>/dev/null &
# Open Chromium in Xephyr.
DISPLAY=:1 chromium-browser &>/dev/null &

It's kind of dirty, but it works extremely well, and I don't have to worry about messing with my monitor setup when I need to give a presentation.

Edit: an earlier version of this post used "Xephyr -bc -ac -noreset -screen 1920x1000 :1 &" for the Xephyr command; I can't get this to work with "-bc" anymore; I must have copied the wrong command when I published the post.