diff --git a/app_state.py b/app_state.py
index 24e7b5a..4dc1c5e 100644
--- a/app_state.py
+++ b/app_state.py
@@ -8,7 +8,7 @@ so later rebinding would only ever be seen by the module that did it.
 import configparser
 import os
 
-VERSION = "0.0.0.139"
+VERSION = "0.0.0.140"
 PROPERTIES_FILE = os.path.join(os.path.dirname(os.path.abspath(__file__)), "editor.properties")
 DEFAULT_GRID_SIZE = 10
 DEFAULT_ADD_TIMESTAMP = True
diff --git a/main.py b/main.py
index bb553a7..91ac383 100644
--- a/main.py
+++ b/main.py
@@ -2435,6 +2435,30 @@ def do_device_disconnect():
     wiresheet.render_components([])
 
 
+def quit_app():
+    """File > Exit and the window's close box: let go of the device before going.
+
+    Leaving without a proper close keeps the device's Sox session and subscriptions
+    alive until it times them out, so the next connect may be turned away.
+    """
+    global _cov_poll_id
+
+    if _cov_poll_id is not None:
+        app_state.window.after_cancel(_cov_poll_id)
+        _cov_poll_id = None
+
+    ds = app_state.device_session
+    app_state.device_session = None
+    app_state.live_mode = False
+    if ds is not None:
+        try:
+            ds.disconnect()
+        except Exception:
+            pass
+
+    app_state.window.destroy()
+
+
 def _start_cov_polling():
     """Starts the 100ms COV event polling loop on the main thread."""
     global _cov_poll_id
@@ -2616,7 +2640,7 @@ menu_file.add_command(label="Save As...", command=sax_file.save_file_as)
 menu_file.add_separator()
 menu_file.add_command(label="Print...", command=wiresheet.print_sheet, accelerator=f"{accel_mod}+P")
 menu_file.add_separator()
-menu_file.add_command(label="Exit", command=window.quit)
+menu_file.add_command(label="Exit", command=quit_app)
 menu_bar.add_cascade(label="File", menu=menu_file)
 
 menu_edit = tk.Menu(menu_bar, tearoff=0)
@@ -3000,6 +3024,8 @@ paned_window.add(frame_editor, weight=3)
 app_state.dirty_display_callback = refresh_dirty_display
 undo.set_refresh_callback(refresh_all_views)
 sax_file.set_tree_filled_callback(on_tree_filled)
+sax_file.set_disconnect_device_callback(do_device_disconnect)
+window.protocol("WM_DELETE_WINDOW", quit_app)
 refresh_palette_kits()
 apply_left_pane_width()
 
diff --git a/sax_file.py b/sax_file.py
index 86b22e8..92baff6 100644
--- a/sax_file.py
+++ b/sax_file.py
@@ -31,6 +31,15 @@ def set_tree_filled_callback(callback):
     _tree_filled = callback
 
 
+_disconnect_device = None
+
+
+def set_disconnect_device_callback(callback):
+    """Registers how to drop the live device session before a file takes its place."""
+    global _disconnect_device
+    _disconnect_device = callback
+
+
 def parse_xml_to_tree(treeview, parent_node, xml_element):
     """Recursively traverses XML elements and adds <comp> tags to the Treeview."""
     for child in xml_element:
@@ -132,19 +141,6 @@ def open_file():
         messagebox.showwarning("Action Blocked", "Cannot load SAX profiles because 'Sedona home folder' configuration is missing or empty.")
         return
 
-    if app_state.live_mode:
-        answer = messagebox.askyesnocancel(
-            "Device Connected",
-            "A live device connection is active.\n\n"
-            "Disconnect from the device and open a file?\n\n"
-            "Choose Yes to disconnect and open, No to cancel.",
-        )
-        if not answer:
-            return
-        # Import here to avoid circular import at module level
-        from main import do_device_disconnect
-        do_device_disconnect()
-
     file_path = filedialog.askopenfilename(
         title="Select SAX File",
         filetypes=[("SAX Files", "*.sax"), ("XML Files", "*.xml"), ("All Files", "*.*")],
@@ -152,6 +148,11 @@ def open_file():
     if not file_path:
         return
 
+    # A session is a file or a device, never both. The device goes only once a file has
+    # actually been chosen, so cancelling the dialog leaves the connection as it was.
+    if app_state.device_session is not None and _disconnect_device is not None:
+        _disconnect_device()
+
     canvas = app_state.canvas
     tree = app_state.tree