Linux: system audio fails, "Could not confirm the Linux audio input route ... but the input did not change" (PipeWire 1.0.5) #3514

Description

Recording with system audio enabled fails on Linux. Cap shows this error:

studio recording actor: Linux system audio could not reach the output monitor: Linux system audio input 'pulse' failed to connect: BuildStream: Could not confirm the Linux audio input route: PulseAudio/PipeWire acknowledged routing stream 1183 to 'alsa_output.pci-0000_00_1f.3.analog-stereo.monitor', but the input did not change

Expected: system audio is captured from the default output's monitor source.

Screen and microphone recording work. Only system audio fails.

Reproduction

1. Launch Cap 0.6.1 on Linux Mint 22.3 MATE (X11, PipeWire).
2. Enable system audio (microphone also enabled).
3. Start a recording.
4. The error above appears.

I also tried making the monitor the default source before launching Cap, so the stream would already start on it:

pactl set-default-source alsa_output.pci-0000_00_1f.3.analog-stereo.monitor

Same error (stream 1404). This makes me suspect the route check fails whenever the input doesn't change, even if the stream is already on the monitor, but that's a guess from outside.

Additional Context

• Cap version: 0.6.1 (.deb, amd64, binary dated 2026-10-20)
• Operating system, version: Linux Mint 22.3 MATE, X11 session
• Device (optional): Laptop, Intel Arrow Lake graphics, built-in Intel HDA audio (pci-0000_00_1f.3)

Audio server (pactl info):

Server Name: PulseAudio (on PipeWire 1.0.5)
Default Sink: alsa_output.pci-0000_00_1f.3.analog-stereo
Default Source: alsa_input.pci-0000_00_1f.3.analog-stereo

Log excerpt (from terminal output):

INFO cap_gpui::main_window: enumerated capture devices cameras=2 microphones=3 displays=1 windows=5
WARN cap_recording::sources::screen_capture::linux: Reconnecting the Linux system-audio input after routing failed error=Linux system audio input 'pulse' failed to connect remaining_retries=1

Note: Cap was launched with GPUI_X11_SCALE_FACTOR=1 as a workaround for #2323 (fixed in #2590, not yet released). I don't expect that to affect audio.