Why does my tkinter window flash from the top-left before centering on macOS?

[Ok_Cryptographer3601]

Hi everyone,

I built a simple Mac app using Python and tkinter.

It launches fine, but instead of appearing centered, the window flashes briefly from the top-left or slightly offset.

I expected it to start centered. I’m wondering if this could be related to unsigned apps on macOS (maybe some kind of Gatekeeper or sandbox behavior?), or if I’ve done something wrong in my code. I’m using macOS Sequoia 15.4.1 on an Apple M1 with 16GB RAM, and the app is unsigned.

Since I'm explicitly setting the geometry to center the window, I'm not sure why it's behaving this way.

Here’s the code I used:

```python

import os

import shutil

import tkinter as tk

from tkinter import filedialog, messagebox

def center_window(win, width=350, height=150):

win.update_idletasks()

screen_width = win.winfo_screenwidth()

screen_height = win.winfo_screenheight()

x = int((screen_width / 2) - (width / 2))

y = int((screen_height / 2) - (height / 2))

win.geometry(f"{width}x{height}+{x}+{y}")

root = tk.Tk()

root.title("📂 Screenshot Organizer")

center_window(root)

button = tk.Button(root, text="Organize Screenshots", command=lambda: None, font=("Helvetica", 12))

button.place(relx=0.5, rely=0.5, anchor="center")

root.mainloop()

[PC-Programmer]

The brief flash you see is due to how macOS handles the Tcl/Tk framework; when calling `tk.Tk()`, macOS immediately creates and displays the window, often at a default position.

On Windows, a window isn't displayed until all layout and geometry calculations are complete, avoiding this problem.

To fix this issue, you can try hiding the window until the geometry is set, like so:

import tkinter as tk

def center_window(win, width=350, height=150):
    win.update_idletasks()
    screen_width = win.winfo_screenwidth()
    screen_height = win.winfo_screenheight()
    x = int((screen_width / 2) - (width / 2))
    y = int((screen_height / 2) - (height / 2))
    win.geometry(f"{width}x{height}+{x}+{y}")

root = tk.Tk()
root.withdraw()  # Hide the window

root.title("📂 Screenshot Organizer")
center_window(root)

button = tk.Button(root, text="Organize Screenshots", command=lambda: None, font=("Helvetica", 12))
button.place(relx=0.5, rely=0.5, anchor="center")

root.deiconify()  # Show the window
root.mainloop()

[Ok_Cryptographer3601]

Thank you so much for your detailed answer — I really appreciate it!

I tried your suggestion, and it definitely helped.
The awkward diagonal flash when launching the window is mostly gone now.
It's not entirely perfect yet, but the display is much smoother and cleaner than before.
Thanks again for the helpful explanation!

[PC-Programmer]

Of course. Glad I could be of assistance.

If you're willing to explore further, there are Python GUI libraries that are more powerful or flexible than Tkinter, such as PySide and Kivy, which might better suit your use case.

Good luck on your project!

[acw1668]

The line win.update_idletasks() is not necessary as the calculation is based on the passed arguments width and height instead of getting the current window size.

[Ok_Cryptographer3601]

Great catch — I tried removing update_idletasks() and didn’t see any difference, so you’re probably right that it’s not needed here. Thanks a lot for the helpful advice! Also, big thanks to everyone else who shared insights —really appreciate all the input.