How to correctly add another panel if a ScrolledPanel is used in wxPython?

Question

In my wxPython 4.1.0 app I inherit from wx.Frame , then create a main panel from wx.Panel , add a ScrolledPanel and to that I add a wx.Bitmap as child. That bitmap/image is larger in height than the scrolled panel, so the vertical scroll bar is shown - everything as expected.

Then I add another wx.Panel below that scrolled panel, without any children, but that alone makes the scroll bar in the scrolled panel disappear. Is this a bug or a feature?

Here is a fully working example.
Uncomment the code in def init_panel(self) to see the strange behaviour:

import wx
import wx.grid
import wx.lib.intctrl
import wx.lib.scrolledpanel

class TestApp(wx.Frame):
    IMAGE_SIZE = 800
    SCROLL_SPEED = 10

    def __init__(self):
        super(TestApp, self).__init__(
            None, title="wxPython Test", style=wx.DEFAULT_FRAME_STYLE ^ wx.RESIZE_BORDER,
        )

        self.init_panel()
        self.main_panel.Layout()
        self.main_panel.Fit()
        self.Center()
        self.Show()

    def init_panel(self):
        self.main_panel = wx.Panel(self)
        self.main_sizer = wx.BoxSizer(wx.VERTICAL)
        self.main_panel.SetSizer(self.main_sizer)

        self.bitmaps_panel = wx.lib.scrolledpanel.ScrolledPanel(self.main_panel)
        self.bitmaps_panel.SetMinSize((-1, 250))
        self.bitmaps_panel.SetupScrolling(scroll_x=False, scroll_y=True, rate_y=self.SCROLL_SPEED)
        self.bitmaps_panel.SetAutoLayout(True)

        self.bitmaps_sizer = wx.BoxSizer(wx.VERTICAL)
        self.bitmaps_panel.SetSizer(self.bitmaps_sizer)

        bitmap = wx.Bitmap(wx.Image(self.IMAGE_SIZE, self.IMAGE_SIZE))
        self.bitmaps_sizer.Add(wx.StaticBitmap(self.bitmaps_panel, wx.ID_ANY, bitmap))
        self.bitmaps_sizer.Fit(self)
        self.main_sizer.Add(self.bitmaps_panel, 0, flag=wx.EXPAND)

        ## With this block commented it works, uncomment to see the scroll bar dissapear
        # self.stats_panel = wx.Panel(self)
        # self.stats_sizer = wx.BoxSizer(wx.VERTICAL)
        # self.stats_panel.SetSizer(self.stats_sizer)
        # self.main_sizer.Add(self.stats_panel, 0, flag=wx.EXPAND)

        self.main_sizer.Fit(self)

if __name__ == "__main__":
    wxapp = wx.App()
    app = TestApp()
    wxapp.MainLoop()

Answer 1

Might this quirk not be explained by your creating stats_panel as a child of self , rather than self.main_panel ?

import wx
import wx.grid
import wx.lib.intctrl
import wx.lib.scrolledpanel

class TestApp(wx.Frame):
    IMAGE_SIZE = 800
    SCROLL_SPEED = 10

    def __init__(self):
        super(TestApp, self).__init__(
            None, title="wxPython Test", style=wx.DEFAULT_FRAME_STYLE)

        self.init_panel()
        self.main_panel.Layout()
        self.main_panel.Fit()
        self.Center()
        self.Show()

    def init_panel(self):
        self.main_panel = wx.Panel(self)

        self.bitmaps_panel = wx.lib.scrolledpanel.ScrolledPanel(self.main_panel)
        self.bitmaps_panel.SetMinSize((-1, 250))
        self.bitmaps_panel.SetupScrolling(scroll_x=True, scroll_y=True, rate_y=self.SCROLL_SPEED)
        self.bitmaps_panel.SetAutoLayout(True)

        bitmap = wx.Bitmap(wx.Image(2000, self.IMAGE_SIZE))
        self.bitmap = wx.StaticBitmap(self.bitmaps_panel, wx.ID_ANY, bitmap)

        self.stats_panel = wx.Panel(self.main_panel)
        self.stats_panel.SetBackgroundColour('lightblue')
        label = wx.StaticText(self.stats_panel, label="Stats Panel")


        self.bitmaps_sizer = wx.BoxSizer(wx.VERTICAL)
        self.bitmaps_sizer.Add(self.bitmap, 0, wx.EXPAND, 5)
        self.bitmaps_panel.SetSizer(self.bitmaps_sizer)

        self.stats_sizer = wx.BoxSizer(wx.VERTICAL)
        self.stats_sizer.Add(label)
        self.stats_panel.SetSizer(self.stats_sizer)

        self.main_sizer = wx.BoxSizer(wx.VERTICAL)
        self.main_sizer.Add(self.bitmaps_panel, 0, flag=wx.EXPAND)
        self.main_sizer.Add(self.stats_panel, 1, flag=wx.EXPAND)
        self.main_panel.SetSizer(self.main_sizer)


if __name__ == "__main__":
    wxapp = wx.App()
    app = TestApp()
    wxapp.MainLoop()