`query_active_config` asked `QueryDisplayConfig` for the active topology even when the
sizing call had just answered `numPaths = 0`. Windows rejects a zero-count query rather
than handing back an empty set, so "nothing is active" came back as "the query failed".
Zero active paths is an ordinary state, not a broken one: every panel off or in standby, a
KVM switched away, a headless box between its adapter arriving and its first monitor. The
`None` propagated into the two places that matter — `count_other_active` reported failure
where the honest answer is `Some(0)`, and `isolate_displays_ccd` bailed at its first line,
which is exactly the teardown-gate condition whose recovery legs exist to stop the
operator's physical panels being left dark (sweep §5 3.1).
MEASURED on .173 (RTX 4090, Win11 26200) with the TV powered off — every monitor devnode
Code 45, `numPaths = 0` for `QDC_ALL_PATHS` too — in a live, logged-on CONSOLE session:
`GetDisplayConfigBufferSizes` returns 0 paths and `QueryDisplayConfig` then returns 0x57
ERROR_INVALID_PARAMETER (0x5 ERROR_ACCESS_DENIED from session 0). This is the mechanism
behind the plan's ⭐ note that `count_other_active(&[])` yields `None` on that box: not the
API refusing us, just a query nobody should have made.
Three changes, one defect:
* `query_active_config` short-circuits `numPaths == 0` to an empty-but-valid config.
* `isolate_displays_ccd` returns that empty snapshot straight away instead of entering its
retry loop. Nothing is active, so nothing needs deactivating and nothing needs restoring;
the loop's re-commit exists to drive the IddCx adapter's COMMIT_MODES, and with no path
at all there is no config to commit — four attempts would each log a rejected apply and
sleep 250 ms to reach the topology we already have. `restore_displays_ccd` already
treated an empty config as the no-op it is, so the round trip stays consistent.
* `set_virtual_primary_ccd` goes through the shared query. It carried a verbatim copy of it
— same flags, same shape — and was therefore the one CCD entry point that would not have
inherited the fix. Same seam asymmetry this crate keeps producing, and two hand-written
unsafe blocks go with it.
Also this file's first tests. The case is `#[ignore]`d hardware, so it reports `ignored`
rather than a vacuous `ok` when nobody runs it. On-glass A/B in the console session on
.173: it passes with the short-circuit and FAILS without it, naming the conflation. A box
that does have a display lit passes either way.
⚠️ The test is read-only and must stay that way — the obvious companion assertion,
`isolate_displays_ccd(&[])`, is deliberately not written: an empty keep set means "keep
nothing", so on a box with displays it would deactivate every one of them.
Verified on .173: `clippy -p punktfunk-host -p pf-vdisplay --all-targets` and
`clippy -p pf-win-display --all-targets` both clean, pf-vdisplay 56 pass, pf-win-display
1 pass + 1 ignored. Both xcheck targets clean.
enricobuehler
ea9a25d3b1