iOS pairing scanner: delayed bounded retry for reason-1 interruption

On device the camera still reported reason 1 (videoDeviceNotAvailable-
InBackground) even when started from viewDidAppear with the app
foreground-active: the camera assertion isn't granted until the sheet's
presentation transition fully settles (a few hundred ms), and the
back-to-back retries all fired inside that unsettled window.

Add a short delayed retry (0.5s, bounded to 12 attempts) driven by the
interruption notification, plus a proactive +0.6s retry after the view
appears, resetting the counter once the session actually starts. Also log
the window/app/scene activation state to confirm the foreground signals.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

NucleicRemote/NucleicRemote/Views/PairingScannerView.swift

@@ -121,6 +121,7 @@ final class QRScannerController: UIViewController, AVCaptureMetadataOutputObject
     /// preview layer and delegate callbacks touch the main thread.
     private let sessionQueue = DispatchQueue(label: "nucleic.qrscanner.session")
     private var configured = false
+    private var startAttempts = 0
     private var didFind = false
 
     private var previewView: CameraPreviewView { view as! CameraPreviewView }
@@ -209,17 +210,24 @@ final class QRScannerController: UIViewController, AVCaptureMetadataOutputObject
     override func viewDidAppear(_ animated: Bool) {
         super.viewDidAppear(animated)
         startSessionIfReady()
+        // The presentation transition can leave the session transiently "backgrounded" even
+        // though the app is foreground; a short delayed retry catches the settled state if the
+        // immediate start was interrupted.
+        DispatchQueue.main.asyncAfter(deadline: .now() + 0.6) { [weak self] in self?.startSessionIfReady() }
     }
 
     /// Start the camera only once it can actually run — configured, on screen, and
     /// foreground-active. Starting during the sheet's presentation made iOS interrupt the
     /// session (`videoDeviceNotAvailableInBackground`, reason 1) and leave it stopped → black.
-    /// Every start trigger (config finished, view appeared, interruption ended, app foregrounded)
+    /// Every start trigger (config finished, view appeared, delayed retry, interruption ended,
-    /// funnels here; the session-queue hop no-ops if already running, so it's idempotent.
+    /// app foregrounded) funnels here; the session-queue hop no-ops if already running.
     @objc private func startSessionIfReady() {
         DispatchQueue.main.async { [weak self] in
-            guard let self, self.isViewLoaded, self.view.window != nil,
+            guard let self, self.isViewLoaded else { return }
-                  UIApplication.shared.applicationState == .active else { return }
+            let appState = UIApplication.shared.applicationState
+            let sceneState = self.view.window?.windowScene?.activationState
+            scannerLog.notice("📷 tryStart window=\(self.view.window != nil, privacy: .public) app=\(appState.rawValue, privacy: .public) scene=\(sceneState?.rawValue ?? -1, privacy: .public)")
+            guard self.view.window != nil, appState == .active else { return }
             self.sessionQueue.async {
                 guard self.configured, !self.session.isRunning else { return }
                 self.session.startRunning()
@@ -231,6 +239,7 @@ final class QRScannerController: UIViewController, AVCaptureMetadataOutputObject
     @objc private func sessionDidStart() {
         DispatchQueue.main.async { [weak self] in
             guard let self else { return }
+            self.startAttempts = 0
             self.report(.scanning)
             let connection = self.previewView.previewLayer.connection
             scannerLog.notice("📷 didStartRunning: previewConnection=\(connection != nil, privacy: .public) active=\(connection?.isActive ?? false, privacy: .public) viewBounds=\(self.view.bounds.debugDescription, privacy: .public)")
@@ -246,6 +255,15 @@ final class QRScannerController: UIViewController, AVCaptureMetadataOutputObject
     @objc private func sessionInterrupted(_ note: Notification) {
         let reason = (note.userInfo?[AVCaptureSessionInterruptionReasonKey] as? Int) ?? -1
         scannerLog.error("📷 sessionInterrupted reason=\(reason, privacy: .public)")
+        // Reason 1 fires transiently while the sheet's presentation settles even though the app
+        // is foreground; back off briefly and retry until it sticks (bounded).
+        DispatchQueue.main.async { [weak self] in
+            guard let self, self.startAttempts < 12 else {
+                scannerLog.error("📷 gave up starting after retries"); return
+            }
+            self.startAttempts += 1
+            DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) { [weak self] in self?.startSessionIfReady() }
+        }
     }
 
     override func viewDidDisappear(_ animated: Bool) {