The error

When trying to launch the beta snap, MarkText fails to open and produces this log message:

nucci@baragon:~$ marktext
[79834:0620/183701.542593:FATAL:zygote_host_impl_linux.cc(117)] No usable sandbox! Update your kernel or see https://chromium.googlesource.com/chromium/src/+/main/docs/linux/suid_sandbox_development.md for more information on developing with the SUID sandbox. If you want to live dangerously and need an immediate workaround, you can try using --no-sandbox.
Trace/breakpoint trap (core dumped)

Solution

Looking at the documentation page referenced, looks like the Chrome developers still have yet to remove the setuid sandbox from Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=598454. To get around this, I will need to modify the apps section of snapcraft.yaml to the following:

apps:
  marktext:
    extensions: [gnome-3-38]
    command: opt/MarkText/marktext --no-sandbox
    desktop: usr/share/applications/marktext.desktop
    environment:
      TMPDIR: $XDG_RUNTIME_DIR
    plugs:
      - shmem
      - gsettings
      - home
      - login-session-observe
      - network
      - network-bind
      - opengl
      - pulseaudio
      - audio-playback
      - audio-record
      - removable-media
      - screen-inhibit-control
      - unity7
      - camera

The addition of the --no-sandbox flag to the MarkText invocation command will deactivate the check by Chromium and allow the snap package to run on other folk's Linux machines.