我有一个选项卡控件，其中标签2.1和标签2.2的滚动条冲突。
Tab 2.2滚动条可以正确地垂直滚动。但是，Tab 2.1的滚动条不能滚动。
我怎样才能解决这个问题呢？

from tkinter import ttk
import tkinter as tk

window = tk.Tk() 
window.attributes('-zoomed', True)

style = ttk.Style(window)

tabControl = ttk.Notebook(window, style='Custom.TNotebook', width=700, height=320)
tabControl.place(x=1, y=1)

#TAB 1
main = ttk.Notebook(tabControl)
tabControl.add(main, text ='Tab 1')

#TAB 2
tab2 = ttk.Notebook(main)
main.add(tab2, text="Tab 2")

##TAB 2.1
a = ttk.Frame(tab2)
tab2.add(a, text="Tab 2.1")

canvas = tk.Canvas(a)

scrollbar = ttk.Scrollbar(a, orient="vertical", command=canvas.yview)
scrollable_frame = ttk.Frame(canvas, width = 800, height = 800)

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(scrollable_frame, width = 18)
combo1.place(x=20, y=20)
combo1['value'] = ["text1", "text2"]

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

##TAB 2.2
b = ttk.Frame(tab2)
tab2.add(b, text="Tab 2.2")

canvas = tk.Canvas(b)

scrollbar = ttk.Scrollbar(b, orient="vertical", command=canvas.yview)
scrollable_frame = ttk.Frame(canvas, width = 800, height = 800)

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(scrollable_frame, width = 18)
combo1.place(x=20, y=20)
combo1['value'] = ["text1", "text2"]

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

window.mainloop()