Scrollbar canvas is displayed but does not scroll down. Conflict of tabs and frames? How do I resolve?

Question

Why doesn't the scrollbar go down to the tab X (in Tab1) where the comboboxes are? The scrollbar is displayed, but it does not go down. There are no errors.

On the other hand, if I set scrollable_frame to comboboxes (as I think you should do it right), the comboboxes disappear and are not displayed

What am I doing wrong? Also is there something to fix in the code? Can you show me the code please? (with comments I may not understand) Thank you

I specify that I use Canvas and that I would like to use the tkinter widgets

import tkinter as tk                    
from tkinter import ttk
  
root = tk.Tk()
root.title("Tab Widget")
root.attributes('-zoomed', True)
tabControl = ttk.Notebook(root, style='Custom.TNotebook', width=400, height=220)
  
tab1 = ttk.Notebook(tabControl)
tab2 = ttk.Notebook(tabControl)
  
tabControl.add(tab1, text ='Tab 1')
tabControl.add(tab2, text ='Tab 2')
tabControl.place(x=1, y=1)

#tab 1
a = ttk.Frame(tab1)
canvas = tk.Canvas(a)

scrollbar = ttk.Scrollbar(a, orient="vertical", command=canvas.yview)
scrollable_frame = ttk.Frame(canvas)

scrollable_frame.bind(
    "<Configure>",
    lambda e: canvas.configure(
        scrollregion=canvas.bbox("all")
    )
)

canvas.create_window((0, 0), window=scrollable_frame, anchor="nw")
canvas.configure(yscrollcommand=scrollbar.set)


combo1=ttk.Combobox(a, width = 18)
combo1.place(x=20, y=20)
combo1['value'] = ["text1", "text2"] 
          
combo2=ttk.Combobox(a, width = 18)
combo2.place(x=20, y=80)
combo2['value'] = ["text1", "text2"] 

combo3=ttk.Combobox(a, width = 18)
combo3.place(x=20, y=140)
combo3['value'] = ["text1", "text2"] 

combo4=ttk.Combobox(a, width = 18)
combo4.place(x=20, y=200)
combo4['value'] = ["text1", "text2"]


a.pack()
canvas.pack(side="left", fill="both", expand=True)
scrollbar.pack(side="right", fill="y")


b = ttk.Frame(tab1)
tab1.add(a, text="X")
tab1.add(b, text="Y")


#tab 2
c = ttk.Frame(tab2)
d = ttk.Frame(tab2)

root.mainloop()