Field-reported 2026-07-23: a consent prompt left up on an unattended host made
the box unreachable. Every connect black-screened and failed after ~60 s, and
the host had to be reached over SSH to clear the prompt.
Windows refuses ChangeDisplaySettingsEx/SetDisplayConfig issued from a thread
that is not on the desktop currently receiving input. While UAC (or the lock /
logon screen) is up that desktop is Winlogon, and the host — which the service
launches explicitly onto WinSta0\Default — got DISP_CHANGE_FAILED /
ERROR_ACCESS_DENIED for every write. A session starting in that window could
never set its virtual display's mode, so the capturer sized its ring to a stale
mode, every composed frame was dropped for a size mismatch, and bring-up burned
8 retries before giving up.
Measured on glass (RTX box, SYSTEM host in console session 2, real consent
prompt up, virtual display active):
INPUT desktop = Winlogon
UNBOUND CDS_TEST -> -1 (DISP_CHANGE_FAILED)
UNBOUND SDC_VALIDATE -> 0x5 (ERROR_ACCESS_DENIED)
BOUND CDS_TEST -> 0 (DISP_CHANGE_SUCCESSFUL)
BOUND SDC_VALIDATE -> 0x0 (ERROR_SUCCESS)
New input_desktop.rs mirrors pf-inject's sendinput.rs retry model: issue the
write, and only when it fails the way a wrong-desktop write fails, rebind this
thread to the current input desktop and retry once. A working write is never
touched, so the normal path is unchanged. The retry predicate stays narrow
(ERROR_ACCESS_DENIED only) so the unrelated 0x57 exclusive-mode topology bug is
not re-issued and mis-attributed. Unlike sendinput's dedicated injector thread,
the binding here is SCOPED — a shared display-write thread left on a Winlogon
desktop that is later destroyed would fail every subsequent write, which is the
wedge this removes.
Applied to all eight SetDisplayConfig sites and both ChangeDisplaySettingsExW
calls in set_active_mode. The isolate site now decides its supplied config once,
outside the write, so a retry re-issues the identical config rather than logging
its escalation twice.
A save is logged, because a silent one is indistinguishable in a field log from
a write that never needed saving — which made the first on-glass verification of
this change inconclusive.
Both ACCESS_DENIED diagnostics now ask which cause applies instead of naming
only the disconnected-RDP one: that phrasing sent this investigation chasing a
phantom RDP session on a host that was already service-launched in the console
session, while a consent prompt was the actual cause.
Verified on glass: connect with a consent prompt up now reaches first frame in
3.0 s (was a black screen then "Connection failed" after ~60 s), with six
"retried bound to it and it applied desktop=Winlogon" lines and rc=0x0
throughout.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
enricobuehlerandClaude Opus 4.8
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