Bug 513536

Summary:
KDE Connect Bluetooth backend causes audio crackling/stuttering in Bluetooth headset (A2DP) under PipeWire

Description:
After updating to KDE Connect 25.12.0-1 on EndeavourOS (Arch Linux-based, KDE Plasma 6.5.4), enabling the Bluetooth backend in KDE Connect settings causes severe audio crackling and stuttering when playing audio through a Bluetooth headset (Sony WH-1000XM4) using A2DP profile with AAC codec in PipeWire. The same issue occurs with the two previous versions, 25.11.80 and 25.11.90.

The issue did not occur with the previous version (KDE Connect 25.08.3-2). Downgrading to 25.08.3-2 resolves the problem completely while keeping the Bluetooth backend enabled.

Symptoms:
- Audio starts normally for a few seconds, then crackling persists for an extended period (cycle of normal → heavy crackling → normal).
- The issue occurs only when the Bluetooth backend is enabled in KDE Connect settings (Backends → Bluetooth checked).
- Disabling the Bluetooth backend immediately resolves the crackling (audio becomes stable, though other KDE Connect features over Wi-Fi remain functional).
- Switching to SBC codec reduces crackling but degrades sound quality significantly.
- No issues with HSP/HFP profiles (mono, low quality).

Workaround:
- Disabling the Bluetooth backend in KDE Connect GUI (Settings > Backends > uncheck Bluetooth) or setting disabled_providers=BluetoothLinkProvider in ~/.config/kdeconnect/config fully resolves the issue while keeping the latest KDE Connect version.
- Downgrading KDE Connect to 25.08.3-2 allows using the Bluetooth backend without issues.

This behavior resembles Bug 415976 (KDE Connect increasing Bluetooth buffer size leading to laggy sound in PulseAudio), but occurs in a modern PipeWire environment. It appears the Bluetooth backend interferes with PipeWire's Bluetooth audio buffering or scheduling.

Steps to Reproduce:
1. Update to KDE Connect 25.12.0-1 on EndeavourOS/Arch Linux with PipeWire.
2. Pair a Bluetooth headset (e.g., Sony WH-1000XM4) and set A2DP Sink with AAC codec.
3. Open KDE Connect settings → Backends → Enable Bluetooth.
4. Play audio (e.g., music or video).
5. Observe crackling after a few seconds.
6. Disable Bluetooth backend → Crackling stops immediately.

Environment:
- OS: EndeavourOS (up to date as of December 2025)
- KDE Plasma Version: 6.5.4
- KDE Frameworks Version: 6.21.0
- Qt Version: 6.10.1
- KDE Connect: 25.12.0-1
- Audio server: PipeWire (with WirePlumber)
- Bluetooth headset: Sony WH-1000XM4 (firmware up to date)
- Codec: AAC (default/high priority)

Additional Information:
- No relevant errors in journalctl for PipeWire/WirePlumber/Bluetooth during the issue.
- Tested multiple buffer/quantum adjustments in PipeWire – no effect when Bluetooth backend is enabled.
- Issue started after KDE Applications update to 25.12.0.

This significantly impacts usability for users relying on high-quality Bluetooth audio. A fix to prevent the Bluetooth backend from interfering with A2DP audio buffering would be appreciated.