GUI: non-blocking disconnect, never close VISA sessions under the worker

Pressing Disconnect with the instruments powered off froze the GUI
(apparent crash, 2026-07-06 15:52): worker.join(5) expired while the
worker sat in multi-second VISA timeouts, then bench.close() ran on the
GUI thread -- each driver close() first writes local() to the dead
device (stacked timeouts) and closing a session under an in-flight read
can also take NI-VISA down natively.

_disconnect now detaches the UI immediately and runs all instrument
teardown on a background thread; if the worker is still stuck in I/O
after a 15s join the sessions are deliberately left open (OS reclaims
them at exit) instead of being closed under a live read. Reconnecting
while a teardown is still releasing the instruments is refused with a
console hint.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

testbench/gui.py

@@ -156,6 +156,7 @@ class TestbenchGUI(tk.Tk):
 
         self.bench: MPPTTestbench | None = None
         self.worker: InstrumentWorker | None = None
+        self._teardown_thread: threading.Thread | None = None
         self._log_file = None
         self._log_writer = None
         self._log_count = 0
@@ -1141,6 +1142,10 @@ class TestbenchGUI(tk.Tk):
     # ── Connection ────────────────────────────────────────────────────
 
     def _connect(self) -> None:
+        if self._teardown_thread and self._teardown_thread.is_alive():
+            self._console("Previous disconnect is still releasing the "
+                          "instruments - try again in a few seconds", "warn")
+            return
         try:
             supply_addr = self._supply_addr.get().strip()
             if supply_addr == "auto":
@@ -1193,16 +1198,9 @@ class TestbenchGUI(tk.Tk):
 
     def _disconnect(self) -> None:
         self._stop_log()
-        if self.worker:
-            self.worker.stop()
-            self.worker.join(timeout=5)
+        worker, bench = self.worker, self.bench
         self.worker = None
-        if self.bench:
-            try:
-                self.bench.close()
-            except Exception:
-                pass
-            self.bench = None
+        self.bench = None   # _poll chain sees both None and ends
 
         self._btn_connect.config(state=tk.NORMAL)
         self._btn_setup.config(state=tk.DISABLED)
@@ -1212,7 +1210,40 @@ class TestbenchGUI(tk.Tk):
         self._load_baud.config(state=tk.NORMAL)
         self._meter_addr.config(state=tk.NORMAL)
         self._status_label.config(text="Disconnected")
+
+        if not worker and not bench:
             self._console("Disconnected")
+            return
+
+        # Instrument teardown runs OFF the GUI thread: with dead/absent
+        # devices every VISA/serial call blocks in multi-second timeouts,
+        # which froze the GUI ("not responding" -> apparent crash) when
+        # this was inline (2026-07-06 15:52).  Closing a VISA session
+        # while the worker is still inside a read can also crash NI-VISA
+        # natively, so if the worker won't exit the sessions are left
+        # open -- the OS reclaims them at process exit.
+        def _teardown():
+            def say(msg, tag=None):
+                try:
+                    self._console(msg, tag) if tag else self._console(msg)
+                except Exception:
+                    pass    # window already destroyed
+            if worker:
+                worker.stop()
+                worker.join(timeout=15)
+            if bench:
+                if worker and worker.is_alive():
+                    say("Instrument worker stuck in an I/O timeout - "
+                        "connections left open (freed on exit)", "warn")
+                else:
+                    try:
+                        bench.close()
+                    except Exception:
+                        pass
+            say("Disconnected")
+
+        self._teardown_thread = threading.Thread(target=_teardown, daemon=True)
+        self._teardown_thread.start()
 
     def _setup_all(self) -> None:
         self._send(Cmd.SETUP_ALL)