title: Expand child elements of wx.GridBagSizer on Panel on wx.Notebook
body: <p>I have a simple application with wx.Notebook, it looks like this:</p>
  
  <p><img src="https://i.stack.imgur.com/Ltxf4.png" alt="enter image description here"></p>
  
  <p>To lay out its child elements, I used wx.GridBagSizer - and it works perfectly. However, I encountered a small but annoying issue. When I expand the window of wx.Notebook, it does not expand all the elements and results in window like this one below:</p>
  
  <p><img src="https://i.stack.imgur.com/IXglZ.png" alt="enter image description here"> </p>
  
  <p>Which is not what I want actually. How should my modify my code to be able to expand all the child elements when I resize the window? Cheers :)</p>
  
  <p>My code is quite simple:</p>
  
  <pre><code>import wx
  import wx.richtext
  
  class MyFrame(wx.Frame):
      def __init__(self):
          wx.Frame.__init__(self, None, title='wx.Notebook')
          book = wx.Notebook(self)
          page = MyPage(parent=book)
          book.AddPage(page, 'Page 1')
          book.AddPage(MyPage(book),'Page 2')
          book.AddPage(MyPage(book),'Page 3')
          self.SetSize(wx.Size(320,250))
          self.Centre()
  
  class MyPage(wx.Panel):
      def __init__(self, parent):
          wx.Panel.__init__(self, parent, size=(0,0))
          # create sizer to add a border
  
          bagSizer = wx.GridBagSizer(15, 10)
  
          mainSizer = wx.BoxSizer(wx.VERTICAL)
          label = wx.StaticText(self, label="Text:")
  
          self.content1 = wx.TextCtrl(self, size=(300, 20))
          self.content2 = wx.richtext.RichTextCtrl(self, size=(300, 100), style=wx.TE_MULTILINE)
  
          bagSizer.Add(self.content1, pos=(0, 0), flag=wx.TOP | wx.LEFT | wx.BOTTOM, border=5)
  
          bagSizer.Add(label, pos=(1, 0), flag=wx.TOP | wx.LEFT | wx.BOTTOM, border=5)
  
          bagSizer.Add(self.content2, pos=(2, 0), flag=wx.TOP | wx.LEFT | wx.BOTTOM | wx.EXPAND, border=5)
  
          self.SetSizer(bagSizer)
  
  app = wx.App(False)
  MyFrame().Show()
  app.MainLoop()
  </code></pre>