WxPython's ScrolledWindow element collapses to minimum size

Question

I am using a Panel within a Frame to display images (the GUI need to switch between multiple panels and hence the hierarchy). As images should be displayed in native size I used ScrolledWindow as the panel parent. The scrolls do appear and work, but it causes the Panel to collapse to minimum size and it needs to be resized using drag&drop every time. Is there a way around this?

Below is a reduced version of the code, which shows the problem:

import os
import wx
from wx.lib.pubsub import pub


class Edit_Panel(wx.PyScrolledWindow):
    def __init__(self, parent):
        super(Edit_Panel, self).__init__(parent)

        # Display size
        width, height = wx.DisplaySize()
        self.photoMaxSize = height - 500

        # Loaded image
        self.loaded_image = None

        # Icons
        self.open_icon_id = 500

        # Generate panel
        self.layout()

    def layout(self):
        self.main_sizer = wx.BoxSizer(wx.VERTICAL)

        divider = wx.StaticLine(self, -1, style = wx.LI_HORIZONTAL)
        self.main_sizer.Add(divider, 0, wx.ALL | wx.EXPAND)

        self.toolbar = self.init_toolbar()
        self.main_sizer.Add(self.toolbar, 0, wx.ALL)

        img = wx.EmptyImage(self.photoMaxSize, self.photoMaxSize)
        self.image_control = wx.StaticBitmap(self, wx.ID_ANY,
                                             wx.BitmapFromImage(img))
        self.main_sizer.Add(self.image_control, 0, wx.ALL | wx.CENTER, 5)

        self.image_label = wx.StaticText(self, -1, style = wx.ALIGN_CENTRE)
        self.main_sizer.Add(self.image_label, 0, wx.ALL | wx.ALIGN_CENTRE, 5)

        self.SetSizer(self.main_sizer)

        fontsz = wx.SystemSettings.GetFont(wx.SYS_SYSTEM_FONT).GetPixelSize()
        self.SetScrollRate(fontsz.x, fontsz.y)
        self.EnableScrolling(True, True)

    def init_toolbar(self):
        toolbar = wx.ToolBar(self)
        toolbar.SetToolBitmapSize((16, 16))

        open_ico = wx.ArtProvider.GetBitmap(wx.ART_FILE_OPEN, wx.ART_TOOLBAR, (16, 16))
        open_tool = toolbar.AddSimpleTool(self.open_icon_id, open_ico, "Open", "Open an Image Directory")
        handler = self.on_open_reference
        self.Bind(event = wx.EVT_MENU, handler = handler, source = open_tool)

        toolbar.Realize()

        return toolbar

    def on_open_reference(self, event, wildcard = None):
        if wildcard is None:
            wildcard = self.get_wildcard()

        defaultDir = '~/'
        dbox = wx.FileDialog(self, "Choose an image to display", defaultDir = defaultDir, wildcard = wildcard, style = wx.OPEN)

        if dbox.ShowModal() == wx.ID_OK:
            file_name = dbox.GetPath()

            # load image
            self.load_image(image = file_name)

        dbox.Destroy()

    def get_wildcard(self):
        wildcard = 'Image files (*.jpg;*.png;*.bmp)|*.png;*.bmp;*.jpg;*.jpeg'
        return wildcard

    def load_image(self, image):
        self.loaded_image = image

        # Load image
        img = wx.Image(image, wx.BITMAP_TYPE_ANY)

        # Label image name
        image_name = os.path.basename(image)
        self.image_label.SetLabel(image_name)

        # scale the image, preserving the aspect ratio
        scale_image = True
        if scale_image:
            W = img.GetWidth()
            H = img.GetHeight()
            if W > H:
                NewW = self.photoMaxSize
                NewH = self.photoMaxSize * H / W
            else:
                NewH = self.photoMaxSize
                NewW = self.photoMaxSize * W / H
            img = img.Scale(NewW, NewH)

        self.image_control.SetBitmap(wx.BitmapFromImage(img))

        # Render
        self.main_sizer.Layout()

        self.main_sizer.Fit(self)
        self.Refresh()

        pub.sendMessage("resize", msg = "")


class Viewer_Frame(wx.Frame):
    def __init__(self, parent, id, title):
        super(Viewer_Frame, self).__init__(parent = parent, id = id, title = title)

        # Edit panel
        self.edit_panel = Edit_Panel(self)

        # Default panel
        self.main_panel = self.edit_panel

        # Render frame
        self.render_frame()

        # Subscription to re-render
        pub.subscribe(self.resize_frame, ("resize"))

    def render_frame(self):
        # Main Sizer
        self.main_sizer = wx.BoxSizer(wx.VERTICAL)

        # Add default sizer
        self.main_sizer.Add(self.main_panel, 1, wx.EXPAND)

        # Render
        self.SetSizer(self.main_sizer)
        self.Show()
        self.main_sizer.Fit(self)
        self.Center()


    def resize_frame(self, msg):
        self.main_sizer.Fit(self)


if __name__ == "__main__":
    app = wx.App(False)
    frame = Viewer_Frame(parent = None, id = -1, title = 'Toolkit')
    app.MainLoop()

Answer 1

You're calling Fit(), so you're explicitly asking the panel to fit its contents, but you don't specify the min/best size of this contents anywhere (AFAICS, there is a lot of code here, so I could be missing something).

If you want to use some minimal size for the panel, just set it using SetMinSize().