feat: surface pty I/O failures instead of freezing silently

Sources/Terminal/TerminalSurface.swift

@@ -275,6 +275,10 @@ class TerminalSurface: NSObject, LocalProcessTerminalViewDelegate {
     var title: String = ""
     var pwd: String?
     var isAlive: Bool { !processExited }
+    /// Set when the pty I/O path broke (dropped input, or output reading
+    /// abandoned). The process may still be running, but this surface can no
+    /// longer be trusted to show or deliver anything — it needs a restart.
+    private(set) var ioFailure: String?
     var onProcessExit: ((TerminalSurface) -> Void)?
     /// The tmux session name, if this terminal is wrapped in tmux.
     var tmuxSessionName: String?
@@ -727,12 +731,42 @@ class TerminalSurface: NSObject, LocalProcessTerminalViewDelegate {
             "processTerminated: surfaceId=\(surfaceId) exitCode=\(exitCode ?? -1)")
         onProcessExit?(self)
     }
+
+    /// Input was dropped on the way to the agent. Keystrokes are lost, so say
+    /// so loudly — a silent write failure looks exactly like a frozen tab.
+    func writeFailed(source: LocalProcessTerminalView, errno: Int32) {
+        ioFailure = "write failed (errno \(errno))"
+        DiagnosticLog.shared.log("surface",
+            "writeFailed: surfaceId=\(surfaceId) errno=\(errno) — input was dropped")
+        notifyIOFailure()
+    }
+
+    /// Reading from the agent was abandoned: the terminal will never show
+    /// another byte from this process even though it keeps running. This is
+    /// the frozen-tab failure mode (#99) — it must never be silent again.
+    func readFailed(source: LocalProcessTerminalView, errno: Int32) {
+        ioFailure = "output stopped (errno \(errno))"
+        DiagnosticLog.shared.log("surface",
+            "readFailed: surfaceId=\(surfaceId) errno=\(errno) — terminal will not update again;" +
+            " restart the tab to recover")
+        notifyIOFailure()
+    }
+
+    private func notifyIOFailure() {
+        guard let ioFailure else { return }
+        NotificationCenter.default.post(
+            name: .deckardSurfaceIOFailure,
+            object: nil,
+            userInfo: ["surfaceId": surfaceId, "reason": ioFailure]
+        )
+    }
 }
 
 // MARK: - Notification Names
 
 extension Notification.Name {
     static let deckardSurfaceTitleChanged = Notification.Name("deckardSurfaceTitleChanged")
+    static let deckardSurfaceIOFailure = Notification.Name("deckardSurfaceIOFailure")
     static let deckardSurfaceClosed = Notification.Name("deckardSurfaceClosed")
     static let deckardNewTab = Notification.Name("deckardNewTab")
     static let deckardNewCodexTab = Notification.Name("deckardNewCodexTab")

Sources/Window/DeckardWindowController.swift

@@ -242,6 +242,7 @@ class DeckardWindowController: NSWindowController, NSSplitViewDelegate {
         NotificationCenter.default.addObserver(self, selector: #selector(themeDidChange(_:)), name: .deckardThemeChanged, object: nil)
         NotificationCenter.default.addObserver(self, selector: #selector(vibrancyDidChange), name: .deckardVibrancyChanged, object: nil)
         NotificationCenter.default.addObserver(self, selector: #selector(quotaDidChange), name: QuotaMonitor.quotaDidChange, object: nil)
+        NotificationCenter.default.addObserver(self, selector: #selector(surfaceIODidFail(_:)), name: .deckardSurfaceIOFailure, object: nil)
         // Show cached quota data immediately if available
         quotaDidChange()
 
@@ -1855,6 +1856,38 @@ class DeckardWindowController: NSWindowController, NSSplitViewDelegate {
             : colors.sidebarBackground.cgColor
     }
 
+    /// A surface's pty I/O broke: input is being dropped, or its output will
+    /// never be read again. The process often keeps running, so without this
+    /// the tab just looks frozen with no explanation (#99). Flag it in the tab
+    /// bar so the state is visible and a restart is an obvious next step.
+    @objc private func surfaceIODidFail(_ note: Notification) {
+        DispatchQueue.main.async { [weak self] in
+            guard let self,
+                  let surfaceIdStr = note.userInfo?["surfaceId"] as? UUID
+                    ?? (note.userInfo?["surfaceId"] as? String).flatMap(UUID.init(uuidString:))
+            else { return }
+            let reason = note.userInfo?["reason"] as? String ?? "pty I/O failure"
+            for workspace in self.workspaces {
+                guard let tab = workspace.tabs.first(where: { $0.surface.surfaceId == surfaceIdStr })
+                else { continue }
+                let errorBadge: TabItem.BadgeState
+                switch tab.kind {
+                case .codex: errorBadge = .codexError
+                case .terminal: errorBadge = .terminalError
+                default: errorBadge = .error
+                }
+                if tab.badgeState != errorBadge {
+                    tab.badgeState = errorBadge
+                    self.rebuildSidebar()
+                    self.rebuildTabBar()
+                }
+                DiagnosticLog.shared.log("surface",
+                    "surfaceIOFailure: workspace=\(workspace.path) tab=\"\(tab.name)\" \(reason)")
+                return
+            }
+        }
+    }
+
     @objc private func quotaDidChange() {
         DispatchQueue.main.async { [weak self] in
             guard let self = self else { return }