wxPython: Fit() works on wxPanel, but not on surrounding wxFrame

[Piet]

Hello,
I am still struggling with a dynamic dialog box.
I have created a dialog box that will show a variable number of
controls depending on a selection made in a ComboBox also present in
the frame. The dialog box is not derived from wxDialog, but from a
wxFrame.
The dialog is initialized as follows:

class myDialog(wxFrame):
    def __init__(self,parent,title):
        wxFrame.__init__(self,parent,-1, title,
style=wxDEFAULT_FRAME_STYLE)
        self.panel = wxPanel(self,-1)
The position of the controls on the wxPanel inside the frame is
controlled by sizers, which are initialized as follows:
        self.MasterSizer=wxBoxSizer(wxVERTICAL)
        self.panel.SetSizer(self.MasterSizer)
        self.panel.SetAutoLayout(True)
        self.MasterSizer.Fit(self.panel)
        self.panel.Fit()
        self.Fit()

To ensure that the size of the window and its layout always take into
account the number of the controls, every function that will change
the number of controls ends with the following threelines:
        self.MasterSizer.RecalcSizes()
        self.panel.Fit()
        self.Fit()

For some reason, only the wxPanel changes its size as expected and
fits nicely around the controls placed on it. The surrounding wxFrame
is always (except at initialization time) slightly larger than the
panel, resulting in an ugly, dark grey border on the right and at the
bottom of the frame. How is that possible?
Thanks in advance for your help
Piet