The mirror hardcoded :0. That held under LightDM, which gave the user session :0. GDM does not —
it keeps :0 for its own greeter and starts the user's Xorg with -displayfd, letting the number be
picked at runtime; on this machine the session lands on :1. So after migrating to Ubuntu Desktop
the mirror failed on every attempt with "Can't open display :0", with a healthy session sitting
one number over.
Resolve by socket ownership: /tmp/.X11-unix/X<n> is owned by whoever runs that X server, so the
socket owned by us is the owner's session and anything else is the greeter's. Falls back to the
lowest socket (a root-run Xorg, as LightDM had) and finally to 0, so it is never worse than the
constant it replaces.
The display is now threaded through rather than read from a module constant, so getFramebufferWidth
measures the display actually being mirrored and both log lines name it.
Found while verifying the XFCE-to-GNOME migration on this machine — the session was up and x11 with
the cookie in the GDM path, and only the display number was wrong.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
pastilhasandClaude Opus 5
3b941f4d1f