python - Why is my Tkinter GUI visually out of focus when opened?

Question

When I open my GUI, I can type in it and do stuff, but the OptionMenu and Button widgets look as if the GUI is out of focus. A picture to demonstrate what I mean: (take a look at the dropdown menus and the buttons)

After I focus on a different app and then click on my GUI again, it has the right colors which should be there if it is in focus. Once again a picture so it is clearer what I mean:

So my question is, does anyone know why this is happening and what I should do so that the GUI is also visually in focus when I open it for the first time?

I know it is bad practice to upload all this code as it's better to upload a minimal reproducible example, but I figured every small detail could be important here as to why this is happening. Therefore, I decided to upload a larger piece of the code I'm using for the GUI.

from tkinter import *
from tkinter import messagebox
from tkinter import font as tkfont

root = Tk()
root.config(background='#009688')
root.title('Contractmaker')

# GUI stuff that takes care of the scrollbar
def on_configure(event):
    canvas.configure(scrollregion=canvas.bbox('all'))

def on_mousewheel(event):
    canvas.yview_scroll(int(event.delta), 'units')

# Create some fonts
bold_font = tkfont.Font(weight='bold')

# Create the actual GUI
canvas = Canvas(root, width=450, height=550)
canvas.config(background='#009688')
canvas.pack(side=RIGHT)

scrollbar = Scrollbar(root, command=canvas.yview)
# scrollbar.pack(side=RIGHT, fill='y')

canvas.configure(yscrollcommand=scrollbar.set)
canvas.bind('<Configure>', on_configure)
canvas.bind_all('<MouseWheel>', on_mousewheel)

frame = Frame(canvas)
frame.config(background='#009688')
canvas.create_window((0,0), window=frame)

labelNaamhuurder = Label(frame, text='Naam huurder', bg='#009688', font=bold_font).grid(row=0, column=0, sticky=W, padx=(30, 0), pady=(15, 0))
naamhuurderr = Entry(frame, textvariable=naamhuurder, relief=FLAT, highlightcolor='#9DCCFD')
naamhuurderr.grid(row=0, column=2, pady=(15, 0))

# All the other rows are the same as the one above so I decided to leave them out 

labelAdresapp = Label(frame, text='Adres appartement', bg='#009688', font=bold_font).grid(row=5, column=0, pady=(15, 0), sticky=W, padx=(30, 0))
appartementen = {'Slotlaan 73', 'Slotlaan 77', 'Albert Cuypstraat 22'}
adresapp.set('Slotlaan 73') # Default option
dropdownMenuhuur = OptionMenu(frame, adresapp, *appartementen)
dropdownMenuhuur.config(width=18)
dropdownMenuhuur.grid(row=5, column=2, pady=(15, 0))

labelTypekamer = Label(frame, text='Type kamer', bg='#009688', font=bold_font).grid(row=6, column=0, pady=(15, 0), sticky=W, padx=(30, 0))
typeKamers = {'Grote kamer', 'Kleine kamer', 'Grote kamer gedeeld'}
typekamer.set('Grote kamer') # Default option
dropdownMenutypekamer = OptionMenu(frame, typekamer, *typeKamers)
dropdownMenutypekamer.config(width=18)
dropdownMenutypekamer.grid(row=6, column=2, pady=(15, 0))

empty = Button(frame, text='Opnieuw', command=clear, font=bold_font)
empty.config(width=10, fg='#009688', borderwidth=0, relief=RAISED)
empty.configure(highlightbackground='#009688')
empty.grid(row=11, column=0, pady=(25, 0), padx=(80, 0))

converter = Button(frame, text='OK', command=contractupdater, font=bold_font)
converter.config(width=10, fg='#009688', borderwidth=2, relief=RAISED)
converter.configure(highlightbackground='#009688')
converter.grid(row=11, column=2, pady=(25, 0), padx=(0, 80))

root.mainloop()

For some more context: this is basically a follow-up question on this question.

Answer 1

I'm not sure if this is the right fix for this issue but it works. So by using root.update() I was able to solve the issue, but this leads to another issue of flashing window changing sizes from default at the start which can also be solved with this,

...
root = tk.Tk()
root.wm_withdraw()  # Hide the window (unmapped)
root.update()       # Update the window when it is hidden
...


...
# Show the window back again just before the mainloop with 1ms delay.
root.after(1, root.deiconify) 
root.mainloop()

This should solve your issue with focus at the start, let me know otherwise.