Fix lockscreen volume mirroring for remote/group players

[pcsokonay]

Hi,
I love your app! You've done an awesome job.
Regards, Peter

PR: Fix lockscreen volume mirroring for remote/group players

Summary

Fixes the volume jump bug in lockscreen hardware volume control when using remote/group players. The system volume slider now shadows the MA player volume without dangerous jumps or desync.

References:

• Original feature: ea7144d (Sync group volume management, hardware volume buttons)
• Original feature introduced unintended behaviour and was pathed: 688b635 (Fix hardware volume in background)

Problem

When the app was backgrounded and the system volume changed (e.g., for YouTube), returning to use hardware buttons for MA volume control caused potentially very large volume jumps — the system volume (e.g., 100%) was mirrored directly to MA, potentially blowing out speakers and killing the cat. The patch in 688b635 stopped the volume observer on pause, but this disabled lockscreen volume control entirely.

Approach

Instead of mirroring system volume values to MA, hardware buttons are now treated as directional step inputs. The system volume slider becomes a "button press detector" that silently resets after each event, while MA volume changes independently in safe increments.

Changes

MainActivity.kt (Kotlin — native layer)

• isMAPlaying guard: Observer suppresses mirroring when MA is not actively streaming (isMusicActive, MODE_NORMAL checks), replacing the stop/start of 688b635
• Observer stays alive across pause/resume: onPause only disables dispatchKeyEvent (foreground key interception); the observer remains active for lockscreen button detection
• Direction + delta signal: Each observer event sends direction (+1/-1) and delta (per-step size mapped to 0-100) to Flutter
• Center-reset with MA shadow: After each event, system volume resets to the position matching estimatedMAVolume, clamped to [1, max-1] so buttons always have room in both directions. The slider visually tracks MA instead of snapping to midpoint
• Reduced ignore window: ignoringVolumeChange guard reduced from 1000ms to 100ms, preventing button presses from being swallowed

lib/main.dart (Flutter — app layer)

• Always-step mode: _setAbsoluteVolume never mirrors the system value directly — steps MA by the native delta in the button direction, making volume jumps structurally impossible
• _lastSteppedVolume tracking: Accumulates volume changes synchronously so rapid presses (hold button) use the correct base instead of stale player.volume
• Play-resume re-sync: When isPlaying transitions true, syncs system volume to MA, closing any drift from the suppression window
• setMAPlayingState: Sends playback state to native layer so the observer can self-guard

lib/services/hardware_volume_service.dart

• Updated onAbsoluteVolumeChange stream type to include direction and delta fields
• Added setMAPlayingState() method to notify native layer of playback state

How it works

1. User presses hardware volume button on lockscreen
2. System volume changes by 1 step → ContentObserver fires
3. Kotlin detects direction and delta, sends to Flutter, resets system to MA-equivalent position (clamped [1, max-1])
4. Flutter steps MA by delta in that direction, tracks accumulated value
5. System slider visually shadows MA position; buttons always work in both directions

Desync scenario (MA=20%, system was changed to 100% while paused):

• Play resumes → system syncs back to 20%
• If sync didn't happen: button press steps MA by ~7%, never jumps to 100%

[CollotsSpot]

Hi Peter, thank you for another pull request! I'm currently working on Android Auto fixes. It's taking considerably longer than usual since I'm reliant on other users for testing—I don't have a car to test with. Your fixes are next on my list!