fix(runtime): prevent Cmd+W close crash during session despawn

docs/ARCHITECTURE.md

@@ -335,7 +335,7 @@ Renderer draws attention border / story overlay
 | `platform/sdl.zig` | SDL3 initialization, window management, HiDPI | `init()`, `createWindow()`, `createRenderer()` | `c` |
 | `input/mapper.zig` | SDL keycodes to VT escape sequences, shortcut detection | `encodeKey()`, modifier helpers | `c` |
 | `c.zig` | C FFI re-exports (SDL3, SDL3_ttf constants) | `SDLK_*`, `SDL_*`, `TTF_*` re-exports | SDL3 system libs (via `@cImport`) |
-| `session/state.zig` | Terminal session lifecycle: PTY, ghostty-vt, process watcher, foreground agent detection, graceful agent teardown at quit | `SessionState`, `AgentKind`, `init()`, `deinit()`, `ensureSpawnedWithDir()`, `render_epoch`, `pending_write`, `detectForegroundAgent()`, `sendTermToForegroundPgrp()`, `drainOutputForMs()` | `shell`, `pty`, `vt_stream`, `cwd`, `font`, xev |
+| `session/state.zig` | Terminal session lifecycle: PTY, ghostty-vt, process watcher, foreground agent detection, graceful agent teardown at quit | `SessionState`, `AgentKind`, `init()`, `despawn()`, `deinit()`, `ensureSpawnedWithDir()`, `render_epoch`, `pending_write`, `detectForegroundAgent()`, `sendTermToForegroundPgrp()`, `drainOutputForMs()` | `shell`, `pty`, `vt_stream`, `cwd`, `font`, xev |
 | `session/notify.zig` | Background notification socket thread and queue; handles status and story notifications | `NotificationQueue`, `Notification` (union: status/story), `startThread()`, `push()`, `drain()` | std (socket, thread) |
 | `session/*` (shell, pty, vt_stream, cwd) | Shell spawning, PTY abstraction, VT parsing, working directory detection | `spawn()`, `Pty`, `VtStream.processBytes()`, `getCwd()` | std (posix), ghostty-vt |
 | `render/renderer.zig` | Scene rendering: terminals, borders, animations, terminal scrollbar painting | `render()`, `RenderCache`, per-session texture management | `font`, `font_cache`, `gfx/*`, `anim/easing`, `app/app_state`, `ui/components/scrollbar`, `c` |

src/app/runtime.zig

@@ -1382,7 +1382,7 @@ pub fn run() !void {
                             }
 
                             // Close the terminal
-                            session.deinit(allocator);
+                            session.despawn(allocator);
                             session_interaction_component.resetView(session_idx);
                             session.markDirty();
 
@@ -1967,7 +1967,7 @@ pub fn run() !void {
                             break :blk null;
                         };
                     }
-                    sessions[idx].deinit(allocator);
+                    sessions[idx].despawn(allocator);
                     session_interaction_component.resetView(idx);
                     sessions[idx].markDirty();
                     compactSessions(sessions, session_interaction_component.viewSlice(), &render_cache, &anim_state);

src/session/state.zig

@@ -144,6 +144,11 @@ pub const SessionState = struct {
         InvalidArgument,
     };
 
+    const WaitContextCleanup = enum {
+        destroy_immediately,
+        defer_if_active,
+    };
+
     pub fn init(
         allocator: std.mem.Allocator,
         slot_index: usize,
@@ -264,6 +269,16 @@ pub const SessionState = struct {
     }
 
     pub fn deinit(self: *SessionState, allocator: std.mem.Allocator) void {
+        self.teardown(allocator, .destroy_immediately);
+    }
+
+    /// Runtime close path used while the event loop is still active.
+    /// Keeps active process wait callbacks valid by deferring context destruction.
+    pub fn despawn(self: *SessionState, allocator: std.mem.Allocator) void {
+        self.teardown(allocator, .defer_if_active);
+    }
+
+    fn teardown(self: *SessionState, allocator: std.mem.Allocator, wait_ctx_cleanup: WaitContextCleanup) void {
         self.pending_write.deinit(allocator);
         self.pending_write = .empty;
         self.quit_capture.deinit(allocator);
@@ -287,9 +302,16 @@ pub const SessionState = struct {
             self.process_watcher = null;
         }
         if (self.process_wait_ctx) |ctx| {
-            // Free unconditionally: deinit runs after the event loop stops, so
-            // processExitCallback will never fire for pending completions.
-            allocator.destroy(ctx);
+            switch (wait_ctx_cleanup) {
+                .destroy_immediately => {
+                    allocator.destroy(ctx);
+                },
+                .defer_if_active => {
+                    if (ctx.completion.state() == .dead) {
+                        allocator.destroy(ctx);
+                    }
+                },
+            }
         }
         self.process_wait_ctx = null;
         // Wrap intentionally: process_generation is a bounded counter and may overflow.
@@ -818,6 +840,39 @@ test "SessionState assigns incrementing ids" {
     try std.testing.expectEqualStrings("1", std.mem.sliceTo(second.session_id_z[0..], 0));
 }
 
+test "despawn keeps active wait context alive until callback reclaims it" {
+    const allocator = std.testing.allocator;
+    const size = pty_mod.winsize{
+        .ws_row = 24,
+        .ws_col = 80,
+        .ws_xpixel = 0,
+        .ws_ypixel = 0,
+    };
+    const notify_sock: [:0]const u8 = "sock";
+
+    var session = try SessionState.init(allocator, 0, "/bin/zsh", size, notify_sock);
+    defer session.deinit(allocator);
+
+    const wait_ctx = try allocator.create(SessionState.WaitContext);
+    wait_ctx.* = .{
+        .session = &session,
+        .generation = 0,
+        .pid = 1,
+        .completion = .{},
+    };
+    wait_ctx.completion.flags.state = @enumFromInt(1);
+
+    session.process_wait_ctx = wait_ctx;
+    session.despawn(allocator);
+    try std.testing.expect(session.process_wait_ctx == null);
+
+    var loop = try xev.Loop.init(.{});
+    defer loop.deinit();
+    var completion: xev.Completion = .{};
+    const action = SessionState.processExitCallback(wait_ctx, &loop, &completion, 0);
+    try std.testing.expectEqual(xev.CallbackAction.disarm, action);
+}
+
 fn makeNonBlocking(fd: posix.fd_t) MakeNonBlockingError!void {
     const flags = try posix.fcntl(fd, posix.F.GETFL, 0);
     var o_flags: posix.O = @bitCast(@as(u32, @intCast(flags)));