Why would the ttk widget only appears after there is a configuration change event and not when the button is pressed?

Can you help me understand why the ttk.Checkbutton widget does not appear after the ttk.Button widget is pressed? The ttk.Checkbutton widget would only appear when there is a configuration change event, for example after moving the sash of the ttk.PanedWindow widget or by resizing the root window. Or, when the height of self.f2 is less than the Checkbutton. This code uses a vertical scroll frame which is saved in the file named scrframe.py

Test Code:

import tkinter as tk
from tkinter import ttk
from PIL import Image, ImageTk
from scrframe import VerticalScrollFrame
class App(ttk.PanedWindow):
def __init__(self, parent, orient="vertical"):
super().__init__(parent, orient=orient)
self.parent = parent
self.after_id = None
self.f1 = ttk.Frame(self, style="Top.TFrame")
self.f2 = VerticalScrollFrame(self, background='green')
self.add(self.f1)
self.add(self.f2)
self.b1 = ttk.Button(self.f1, text="Button", command=self.load_image,
width=30)
self.b1.grid(row=0, column=0, sticky="nsew", padx=5, pady=5)
self.f1.bind("<Configure>", self.schedule_event)
self.f2.bind("<Configure>", self.schedule_event)
self.bind("<<SashMoved>>", self.do_something)
# Event handlers
def schedule_event(self, event):
if self.after_id:
self.after_cancel(self.after_id)
self.after_id = self.after(500, self.event_generate, "<<SashMoved>>")
def do_something(self, event):
print("do_something was called")
def load_image(self):
im = '/home/user/Pictures/image.jpeg'
im = Image.open(im)
im.thumbnail((200, 200))
self.thumbnail = ImageTk.PhotoImage(im)
self.thumbnail.image = im
self.cb = ttk.Checkbutton(self.f2.interior, text="Image",
image=self.thumbnail, compound="top")
self.cb.grid(row=0, column=0, padx=5, pady=5)
self.update_idletasks()  # Does not work
if __name__ == '__main__':
root = tk.Tk()
ss = ttk.Style()
ss.configure('Top.TFrame', background="red")
app = App(root)
app.pack(fill="both", expand=True)
root.mainloop()

Issue: Checkbutton does not appears immediately when the height of self.f2 is greater than the Checkbutton.

Checkbutton appears immediately when the height of self.f2 is less than the Checkbutton:

Thanks to the comments by @acw1668. It looks like the issue stems from the ._configure_canvas event handler in scrframe.py simply using canvas.winfo_reqwidth(). Below are printed-out values after the Button is released:

def _configure_interior(event):
interior.winfo_reqwidth()=228 interior.winfo_reqheight()=165
canvas.winfo_reqwidth()=1 canvas.winfo_reqheight()=223
event.width=261 event.height=165
do_something was called

They showed that previously the width of the "canvas window objected" that contained self.interior (a ttk.Frame widget) was mistakenly set to 1 pixels width since canvas.winfo_reqwidth()=1 hence the ttk.Checkbutton widget was hidden.

After further testing, I discovered that simply using event.width is not a robust solution. In the situation when the Tk window width is less than the width of the Checkbutton, the appearance of the Checkbutton will be truncated when the Canvas is scrolled horizontally.

Hence, the correct width to use for the "canvas window objected" should instead be the max of canvas.winfo_reqwidth() and event.width.

def _configure_canvas(event):
print(f"\ndef _configure_canvas(event):")
print(f"{interior.winfo_reqwidth()=} {interior.winfo_reqheight()=}")
print(f"{canvas.winfo_reqwidth()=} {canvas.winfo_reqheight()=}")
print(f"{event.width=} {event.height=}")
if interior.winfo_reqwidth() != canvas.winfo_width():
# update the canvas window object's width to the max of canvas.winfo_reqwidth() and event.width
maxwidth = max([canvas.winfo_reqwidth(), event.width])
canvas.itemconfigure(interior_id, width=maxwidth)

Outcome from using event.width:

Outcome from using max([canvas.winfo_reqwidth(), event.width]):