wxpython: nested sizers, and little square in top left corner?

I'm trying to look into wxPython and sizers, and I put together the following example:

import wx

class MyTestFrame(wx.Frame):
    def __init__(self, parent, title):
        super(MyTestFrame, self).__init__(parent, title=title, 
            size=(250, 150))

        # the master panel of the frame - "Add a panel so it looks correct on all platforms"
        self.panel = wx.Panel(self, wx.ID_ANY)
            # self.panel.SetBackgroundColour(wx.Colour(124, 224, 124)) # to confirm the square is the panel


        # want these buttons absolutely positioned
        btn_A = wx.Button(self, id=1, label='A', pos=(10, 10), size=(30, 30))
        btn_A.SetBackgroundColour(wx.Colour(224, 124, 124))
        btn_B = wx.Button(self, id=2, label='B', pos=(45, 10), size=(30, 30))
        btn_C = wx.Button(self, id=3, label='C', pos=(80, 10), size=(30, 30))

        # additional object

        mastersizer = wx.BoxSizer(wx.VERTICAL)
        btnsizer = wx.BoxSizer(wx.HORIZONTAL)

        btnsizer.Add(btn_A, 0)
        btnsizer.Add(btn_B, 0)
        btnsizer.Add(btn_C, 0)

        mastersizer.Add(btnsizer, 1, wx.EXPAND)
        self.panel.SetSizer(mastersizer)
        #~ mastersizer.Fit(self) # makes the window as large as the buttons

        self.Centre()
        self.Show()


if __name__ == '__main__':
    app = wx.App()
    MyTestFrame(None, 'Test')
    app.MainLoop()

When I run this, I get a window like on the image:

Can someone explain, why do I get that gray little square in the upper left corner - and what would be the correct way to implement the code? (The button is deliberately colored, so it can be obvious.. ) I'm on Ubuntu Lucid, in case this is platform specific.

EDIT: That square is apparently the panel itself, but then I cannot tell why doesn't it resize and become the 'parent' for the buttons, as intended?

Ah well - wasn't that bad; but wasn't that obvious to me :) So here's some reference for other noobs like myself: In the above code, the buttons are defined as children of the frame - not of the panel; so the only change is this:

    # want these buttons absolutely positioned
    # must be children of panel - if panel is to encompass them! 
    btn_A = wx.Button(self.panel, id=1, label='A', pos=(10, 10), size=(30, 30))
    btn_A.SetBackgroundColour(wx.Colour(224, 124, 124))
    btn_B = wx.Button(self.panel, id=2, label='B', pos=(45, 10), size=(30, 30))
    btn_C = wx.Button(self.panel, id=3, label='C', pos=(80, 10), size=(30, 30))

And then all seems fine:

Well, sorry to have wasted space here - but hopefully, may be of use to others :)

Cheers!