fixes

Shared/Audio/AudioPlayer.swift

@@ -89,6 +89,7 @@ class AudioPlayer: NSObject, ObservableObject {
     private var offlineVisBuffer = VisFrameBuffer.empty
     private var offlineVisTimer: Timer?
     private var offlineVisFPS: Double = 30.0
+    private var analysisTask: Task<Void, Never>?
     #endif
     
     // MARK: - Now Playing Artwork
@@ -133,11 +134,14 @@ class AudioPlayer: NSObject, ObservableObject {
     private func suspendVisTimers() {
         stopOfflineVisTimer()
         stopLevelTimer()
-        // Remove the periodic time observer — it fires at 10Hz updating @Published currentTime,
-        // which triggers SwiftUI re-evaluation of the entire view tree in background.
-        // This is the primary driver of the 93% background CPU that kills the process.
+        // Cancel any in-progress offline vis analysis — it's a CPU-intensive FFT
+        // loop that will otherwise run to completion in background, burning ~60% CPU
+        analysisTask?.cancel()
+        analysisTask = nil
+        // Remove the periodic time observer — it fires at 10Hz updating @Published
+        // currentTime, driving SwiftUI re-evaluation of the entire view tree in background
         removeTimeObserver()
-        alog("Background: timers + time observer suspended")
+        alog("Background: timers + time observer + analysis task suspended")
     }
 
     private func resumeVisTimers() {
@@ -1231,7 +1235,8 @@ class AudioPlayer: NSObject, ObservableObject {
         let points = VisualizerSettings.shared.nowPlaying.numberOfPoints
         let cutoff = VisualizerSettings.shared.frequencyCutoff
         
-        Task {
+        analysisTask?.cancel()  // cancel any in-flight analysis from the previous song
+        analysisTask = Task {
             let storage = VisualizerStorageManager.shared
             let hasCache = await storage.hasCache(for: songId)
             
@@ -1504,6 +1509,8 @@ class AudioPlayer: NSObject, ObservableObject {
         nowPlayingSyncTimer = nil
         #if os(iOS)
         stopOfflineVisTimer()
+        analysisTask?.cancel()
+        analysisTask = nil
         if isUsingCrossfade {
             SmartCrossfadeManager.shared.stop()
             isUsingCrossfade = false

iOS/Views/Visualizer/OfflineAudioAnalyzer.swift

@@ -104,6 +104,9 @@ actor OfflineAudioAnalyzer {
         var nextFrameSample = 0   // the audio sample index at which to take the next vis frame
 
         while file.framePosition < totalFrames {
+            // Cooperatively cancel if the app backgrounded mid-analysis
+            try Task.checkCancellation()
+
             let toRead = min(AVAudioFrameCount(readChunkSamples),
                              AVAudioFrameCount(totalFrames - file.framePosition))
             readBuffer.frameLength = 0
@@ -138,6 +141,8 @@ actor OfflineAudioAnalyzer {
             let chunkEnd = chunkStart + chunkLen
 
             while nextFrameSample < chunkEnd {
+                // Check cancellation every frame — the inner loop is the hot path
+                try Task.checkCancellation()
                 // We need fftSize samples ending at nextFrameSample + fftSize/2
                 // (centre the FFT window on the frame position for better transient response)
                 let windowStart = nextFrameSample - fftSize / 2