Tkinter, tkMessagebox and overrideredirect

[marcoberi at gmail.com]

Hi everybody.

I have this code snippet that shows a window without a titlebar (using
overrideredirect) and two buttons on it: one quits and the other one
brings up a simple tkMessageBox.
On Windows (any flavour) the tkMessagebox brings up over the
underlying window.
On Linux (apparently any flavour) the tkMessagebox brings up under the
underlying window.
You can drag the popup window near the main window to discover if it's
over or under it.
Obviously I would like to show a popup that is over the main window!
Before asking I tried, I read, I googled, I pulled my hair off, but no
avail...
Any hints?
Thanks a lot for your time.
Ciao.
Marco.

import tkMessageBox
from Tkinter import *

class App():
    def __init__(self):
        self.root = Tk()
        self.root.overrideredirect(1)
        self.frame = Frame(self.root, width=320, height=200,
                           borderwidth=5, relief=RAISED)
        self.frame.pack_propagate(False)
        self.frame.pack()
        self.bQuit = Button(self.frame, text="Quit",
command=self.root.quit)
        self.bQuit.pack(pady=20)
        self.bHello = Button(self.frame, text="Hello",
command=self.hello)
        self.bHello.pack(pady=20)

    def hello(self):
        tkMessageBox.showinfo("Popup", "Hello!")

app = App()
app.root.mainloop()