Skip a USB headset dongle while no headset is linked to it

[jerrison]

The problem

A wireless headset dongle publishes its CoreAudio devices whenever it is plugged in, whether or not the headset is powered on and linked to it. Auto-switching ranks by presence, so the dongle wins and audio goes into a dead link — it shows as Active while nothing comes out. The only recovery is picking another device by hand every time the headset is off, which is the problem priorities are meant to solve.

CoreAudio gives you nothing to work with here. Measured on a Jabra Link 390, headset off and then on:

Signal	Value	Usable?
kAudioDevicePropertyJackIsConnected	never published by the driver	No
DeviceIsAlive	1 either way	No
Channel counts	2 out / 1 in either way	No
Available sample rates	16k/32k/48k out, 16k in either way	No
Stream format, terminal type	48kHz 2ch, hdph / hmic either way	No
CanBeDefaultDevice	1 either way	No

The fix

The dongle does report it over HID. WirelessLinkMonitor reads that bit for dongles it recognises, and applyHighestPriorityInput/applyHighestPriorityOutput skip a device whose link is down.

For the Jabra Link 390 it is HID Telephony usage 0x2A ("Line"), report 0x04 byte 1 bit 3, which reads 1 only while the headset is linked to the dongle. Adding another dongle is one entry in WirelessLinkMonitor.knownDongles.

That the bit is link state and not a button press was confirmed against the hardware: powering the headset on over Bluetooth left it at 0 while the headset's Bluetooth devices appeared, and when the headset later moved its link to the dongle it went to 1 and the Bluetooth devices were removed 1.7 seconds later.

Why this should not affect anyone else

• Unrecognised devices are always reported usable. The monitor only ever returns down for a dongle in knownDongles whose bit it actually read. Every other device takes exactly the path it takes today.
• No new permissions. Reading the bit needs no Input Monitoring grant and works from an unsigned build — verified.
• Debounced. A link must stay down 3 seconds before the device is skipped, so a momentary drop cannot pull audio away mid-call.
• Backstop poll. HID reports drive it; a 2-second poll covers a missed report so a device cannot stay stuck.

Verified on the hardware

A/B on identical state — dongle plugged in, headset not linked, dongle at priority 1, default output seeded to the dongle before each run:

Build	Result
main	keeps Jabra Link 390 — the dead link
this branch	moves off it to the next usable device

Also checked: with the headset linked the dongle is selected normally, and xcodebuild -scheme AudioPriorityBar -configuration Release succeeds with no new warnings.

UI

A device in this state is greyed with an antenna.radiowaves.left.and.right.slash icon and a not linked label, so a skipped device does not read as a bug. It follows the existing wifi.slash / nosign status-icon pattern.

Notes

• One new file, Services/WirelessLinkMonitor.swift, registered in the Xcode project (the project uses a classic group list, so project.pbxproj needed the four usual entries).
• Tested on macOS 26.5.2, Apple silicon, with a Jabra Link 390 and an Evolve2 75. I only have the one dongle family to test against, so the knownDongles list starts with one entry.