diff -r ed694938c61a Lib/idlelib/Debugger.py
--- a/Lib/idlelib/Debugger.py	Tue Sep 22 13:08:42 2015 +0300
+++ b/Lib/idlelib/Debugger.py	Tue Sep 22 15:30:29 2015 -0700
@@ -71,6 +71,7 @@
         if self.interacting:
             self.top.bell()
             return
+        self.abort_loop()
         if self.stackviewer:
             self.stackviewer.close(); self.stackviewer = None
         # Clean up pyshell if user clicked debugger control close widget.
@@ -191,7 +192,8 @@
             b.configure(state="normal")
         #
         self.top.wakeup()
-        self.root.mainloop()
+        # nested main loop
+        self.root.tk.call('vwait', '::idledebugwait')
         #
         for b in self.buttons:
             b.configure(state="disabled")
@@ -215,23 +217,26 @@
 
     def cont(self):
         self.idb.set_continue()
-        self.root.quit()
+        self.abort_loop()
 
     def step(self):
         self.idb.set_step()
-        self.root.quit()
+        self.abort_loop()
 
     def next(self):
         self.idb.set_next(self.frame)
-        self.root.quit()
+        self.abort_loop()
 
     def ret(self):
         self.idb.set_return(self.frame)
-        self.root.quit()
+        self.abort_loop()
 
     def quit(self):
         self.idb.set_quit()
-        self.root.quit()
+        self.abort_loop()
+
+    def abort_loop(self):
+        self.root.tk.call('set', '::idledebugwait', '1')
 
     stackviewer = None