fix(video): outputs started twice when the 50 Hz switch posted a screen change

The mode switch posts didChangeScreenParameters synchronously; reconcile ran
nested before the output was registered and opened a second window/receiver,
which stayed visible after stopping. Register first, never nest reconcile,
handle the notification asynchronously. Switching sources keeps the 50 Hz mode.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

App/Sources/OutputController.swift

@@ -21,7 +21,8 @@ final class ActiveOutput {
     let receiver: NDIReceiver
     let window: OutputWindow
     let displayID: CGDirectDisplayID
-    let originalMode: CGDisplayMode?
+    /// Mode before Musubi switched to 50 Hz; restored when the output ends.
+    var originalMode: CGDisplayMode?
     var snapshot: NDIReceiver.Snapshot
 
     init(source: Source, receiver: NDIReceiver, window: OutputWindow, displayID: CGDirectDisplayID, originalMode: CGDisplayMode?) {
@@ -57,7 +58,8 @@ final class OutputController {
     init() {
         refreshScreens()
         NotificationCenter.default.addObserver(forName: NSApplication.didChangeScreenParametersNotification, object: nil, queue: .main) { [weak self] _ in
-            MainActor.assumeIsolated { self?.screensChanged() }
+            // Deferred: the notification can arrive synchronously inside a display mode switch.
+            DispatchQueue.main.async { MainActor.assumeIsolated { self?.screensChanged() } }
         }
         timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { [weak self] _ in
             MainActor.assumeIsolated { self?.updateSnapshots() }
@@ -98,8 +100,25 @@ final class OutputController {
         onChange?()
     }
 
+    private var reconciling = false
+    private var reconcileAgain = false
+
     /// Starts and stops outputs so that connected screens match `desired`.
+    /// Never nested: display mode switches post screen-change notifications while we are in here.
     private func reconcile() {
+        guard !reconciling else {
+            reconcileAgain = true
+            return
+        }
+        reconciling = true
+        defer { reconciling = false }
+        repeat {
+            reconcileAgain = false
+            reconcileOnce()
+        } while reconcileAgain
+    }
+
+    private func reconcileOnce() {
         let screens = NSScreen.screens
         let resolved = ScreenMatching.resolve(desired, in: screens.map(\.localizedName))
         let wanted = Dictionary(uniqueKeysWithValues: resolved.map { (screens[$0.key].displayID, $0.value) })
@@ -136,22 +155,32 @@ final class OutputController {
 
     private func start(_ source: Source, on displayID: CGDirectDisplayID) {
         guard let screen = NSScreen.screens.first(where: { $0.displayID == displayID }) else { return }
-        halt(displayID)
+
+        // Switching sources on a screen keeps its 50 Hz mode instead of bouncing to 60 Hz and back.
+        let previous = outputs.removeValue(forKey: displayID)
+        previous?.receiver.stop()
 
         let window = OutputWindow(screen: screen)
         let sink = window.sink
         let receiverName = "Musubi \(screen.localizedName)"
         guard let receiver = NDIReceiver(source: source, receiverName: receiverName, onFrame: { sink.display($0) }) else {
             Log.ndi.error("Could not create receiver for \(source.ndiName, privacy: .public)")
+            previous?.window.orderOut(nil)
+            if let original = previous?.originalMode { DisplayModeSwitcher.restore(original, on: displayID) }
             return
         }
-
-        let mode = DisplayModeSwitcher.setRefreshRate(Self.targetRefreshRate, on: displayID)
         window.onCancel = { [weak self] in self?.stop(displayID) }
         window.setFrame(screen.frame, display: true)
         window.orderFrontRegardless()
+        previous?.window.orderOut(nil)
+
+        // Register before the mode switch: it posts a screen-change notification.
+        let output = ActiveOutput(source: source, receiver: receiver, window: window, displayID: displayID, originalMode: previous?.originalMode)
+        outputs[displayID] = output
+        let mode = DisplayModeSwitcher.setRefreshRate(Self.targetRefreshRate, on: displayID)
+        if output.originalMode == nil { output.originalMode = mode.original }
+        window.setFrame(screen.frame, display: true)
 
-        outputs[displayID] = ActiveOutput(source: source, receiver: receiver, window: window, displayID: displayID, originalMode: mode.original)
         Log.ndi.info("Output \(source.ndiName, privacy: .public) → \(screen.localizedName, privacy: .public) @ \(mode.refreshRate) Hz")
         updateSleepBlocker()
         refreshScreens()