require 'gtk2'

Gtk.init

textview = Gtk::TextView.new
textview.signal_connect('focus-in-event') { |w, event|
    textview.buffer.select_range(textview.buffer.get_iter_at_offset(0), textview.buffer.get_iter_at_offset(-1))
    false
}
textview.buffer.text = 'foobar'

b = Gtk::Button.new('foo')

vb = Gtk::VBox.new
vb.pack_start(textview, true, true)
vb.pack_end(b, false, false)

w = Gtk::Window.new
w.add(vb)
w.set_default_size(300, 300)
w.window_position = Gtk::Window::POS_CENTER
w.show_all

b.grab_focus

Gtk.main