When running the following script, the textview widget is not shown in the window. Interestingly, the bug disappears if a size is requested for the window.

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#!/usr/bin/python

import gtk

class HelloWorldWindow:
    def __init__(self):
        self.window = gtk.Window(gtk.WINDOW_TOPLEVEL)

        # The bug disappears when this line is uncommented.
        #self.window.set_size_request(200, 200)

        textview = gtk.TextView()
        textbuffer = textview.get_buffer()
        textbuffer.set_text('Hello, World!')
        self.window.add(textview)

        self.window.show_all()
        self.window.unmap()
        self.window.map()

if __name__ == "__main__":
    window = HelloWorldWindow()
    gtk.main()