Connect to another device without disconnecting first

Connect and Discover stay enabled while a device is connected. Pressing
Connect in the dialog drops the current device before the new one is
tried; cancelling the dialog leaves the connection alone.

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

connect_dialog.py

@@ -21,8 +21,9 @@ import app_state
 class ConnectDialog:
     """Modal dialog for entering device connection parameters."""
 
-    def __init__(self, parent, on_connect_callback):
+    def __init__(self, parent, on_connect_callback, on_before_connect=None):
         self._callback = on_connect_callback
+        self._before_connect = on_before_connect
         self._thread = None
         self._cancelled = False
 
@@ -103,6 +104,11 @@ class ConnectDialog:
             self._status_label.config(text="Invalid port number", foreground="#c00000")
             return
 
+        # Runs here, on the main thread, rather than in the worker: it is where a
+        # device already connected is let go, and that takes the UI down with it.
+        if self._before_connect is not None:
+            self._before_connect()
+
         self._btn_connect.config(state="disabled")
         self._status_label.config(text="Connecting...", foreground="#666666")
 
@@ -169,9 +175,10 @@ class DiscoverDialog:
     """Modal dialog that broadcasts DASP DISCOVER, lists responding
     devices, and lets the user pick one to connect to."""
 
-    def __init__(self, parent, on_connect_callback):
+    def __init__(self, parent, on_connect_callback, on_before_connect=None):
         self._parent = parent
         self._callback = on_connect_callback
+        self._before_connect = on_before_connect
         self._discover_thread = None
 
         self._dlg = tk.Toplevel(parent)
@@ -318,7 +325,7 @@ class DiscoverDialog:
         host = values[0]
         port = self._port_var.get().strip()
         self._dlg.destroy()
-        _CredentialsDialog(self._parent, self._callback, host, port)
+        _CredentialsDialog(self._parent, self._callback, host, port, self._before_connect)
 
 
 # ────────────────────────────────────────────────────────────────────
@@ -328,8 +335,9 @@ class DiscoverDialog:
 class _CredentialsDialog:
     """Small modal that asks for username and password, then connects."""
 
-    def __init__(self, parent, on_connect_callback, host, port):
+    def __init__(self, parent, on_connect_callback, host, port, on_before_connect=None):
         self._callback = on_connect_callback
+        self._before_connect = on_before_connect
         self._host = host
         self._port = port
         self._thread = None
@@ -397,6 +405,11 @@ class _CredentialsDialog:
             self._status_label.config(text="Invalid port", foreground="#c00000")
             return
 
+        # Runs here, on the main thread, rather than in the worker: it is where a
+        # device already connected is let go, and that takes the UI down with it.
+        if self._before_connect is not None:
+            self._before_connect()
+
         self._btn_connect.config(state="disabled")
         self._status_label.config(text="Connecting...", foreground="#666666")