Firefox RTC screencasting not working

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1

I’m on an Arch base system with home-manager based config and my window-manager is sway. I’m able to get cameras and microphones to work for webRTC in Firefox, but not screen sharing.

I see that when Firefox tries to start a screencast it does so via D-Bus and I can see the following on dbus-monitor

method call time=1686555522.779241 sender=:1.11 -> destination=org.freedesktop.DBus serial=260 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=AddMatch
   string "type='signal',sender='org.freedesktop.portal.Desktop',interface='org.freedesktop.DBus.Properties',member='PropertiesChanged',path='/org/freedesktop/portal/desktop',arg0='org.freedesktop.portal.ScreenCast'"
method return time=1686555522.779267 sender=org.freedesktop.DBus -> destination=:1.11 serial=213 reply_serial=260
method call time=1686555522.779274 sender=:1.11 -> destination=org.freedesktop.DBus serial=261 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=AddMatch
   string "type='signal',sender='org.freedesktop.portal.Desktop',interface='org.freedesktop.portal.ScreenCast',path='/org/freedesktop/portal/desktop'"
method return time=1686555522.779284 sender=org.freedesktop.DBus -> destination=:1.11 serial=214 reply_serial=261
method call time=1686555522.779289 sender=:1.11 -> destination=org.freedesktop.DBus serial=262 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=AddMatch
   string "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='NameOwnerChanged',path='/org/freedesktop/DBus',arg0='org.freedesktop.portal.Desktop'"
method return time=1686555522.779299 sender=org.freedesktop.DBus -> destination=:1.11 serial=215 reply_serial=262
method call time=1686555522.779326 sender=:1.11 -> destination=org.freedesktop.DBus serial=263 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=StartServiceByName
   string "org.freedesktop.portal.Desktop"
   uint32 0
method return time=1686555522.779340 sender=org.freedesktop.DBus -> destination=:1.11 serial=216 reply_serial=263
   uint32 2
method call time=1686555522.785682 sender=:1.11 -> destination=org.freedesktop.DBus serial=264 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=GetNameOwner
   string "org.freedesktop.portal.Desktop"
method return time=1686555522.785710 sender=org.freedesktop.DBus -> destination=:1.11 serial=217 reply_serial=264
   string ":1.413"
method call time=1686555522.790069 sender=:1.11 -> destination=:1.413 serial=265 path=/org/freedesktop/portal/desktop; interface=org.freedesktop.DBus.Properties; member=GetAll
   string "org.freedesktop.portal.ScreenCast"
error time=1686555522.790378 sender=:1.413 -> destination=:1.11 error_name=org.freedesktop.DBus.Error.InvalidArgs reply_serial=265
   string "No such interface “org.freedesktop.portal.ScreenCast”"
method call time=1686555522.790914 sender=:1.11 -> destination=:1.413 serial=266 path=/org/freedesktop/portal/desktop; interface=org.freedesktop.portal.ScreenCast; member=CreateSession
   array [
      dict entry(
         string "session_handle_token"
         variant             string "webrtc_session1216757917"
      )
      dict entry(
         string "handle_token"
         variant             string "webrtc_1255023056"
      )
   ]
error time=1686555522.791143 sender=:1.413 -> destination=:1.11 error_name=org.freedesktop.DBus.Error.UnknownMethod reply_serial=266
   string "No such interface “org.freedesktop.portal.ScreenCast” on object at path /org/freedesktop/portal/desktop"

I’m at a bit of a loss as to what exactly might be causing the no such interface errors. I have xdg-desktop-portal.service and xdg-desktop-portal-wlr.service running with WAYLAND_DISPLAY=wayland-1 and XDG_CURRENT_DESKTOP=sway in the environment.

System logs don’t have anything of relevance.

2

Found the solution. It was only looking for portals in the nix store path of the libexec binary itself. Setting XDG_DESKTOP_PORTAL_DIR works.