`vmgui` sent the child's stderr to /dev/null, so when the X display was
unreachable (the common case: sudo -u ci-runner loses the desktop user's
XAUTHORITY and xhost wasn't granted) the GUI died with "cannot open display"
and the command still printed success — "nothing opens".
Now it:
- pre-flights the display with xdpyinfo and aborts with the exact
`xhost +SI:localuser:ci-runner` hint instead of spawning a doomed process;
- drops an inherited XAUTHORITY that this user can't read (falls back to
host-based xhost auth);
- logs the GUI's stdio to /tmp/vmgui-*.log and grace-polls 1.5s, surfacing the
log if the GUI exits immediately;
- errors (non-zero) when no DISPLAY is resolvable.
Runbook §4.7 updated with the "if nothing opens" guidance. 13 tests; green ≥90%.
Simone
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