bpo-23544: Disable IDLE Stack Viewer when running user code (GH-17163)

Starting stack viewer when user code is running, including when Debugger is active, hangs or crashes IDLE.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
diff --git a/Lib/idlelib/pyshell.py b/Lib/idlelib/pyshell.py
index 0407ca9..0ee2254 100755
--- a/Lib/idlelib/pyshell.py
+++ b/Lib/idlelib/pyshell.py
@@ -989,6 +989,10 @@ def open_debugger(self):
         self.showprompt()
         self.set_debugger_indicator()
 
+    def debug_menu_postcommand(self):
+        state = 'disabled' if self.executing else 'normal'
+        self.update_menu_state('debug', '*tack*iewer', state)
+
     def beginexecuting(self):
         "Helper for ModifiedInterpreter"
         self.resetoutput()