From https://stackoverflow.com/questions/36336010/cannot-see-all-tabs-in-ttk-notebook

import tkinter as tk
from tkinter.ttk import Frame, Notebook

root = tk.Tk()
nb = Notebook(root, width=320, height=240)
nb.pack(fill='both', expand=1)
page0 = Frame(nb)
page1 = Frame(nb)
page2 = Frame(nb)
page3 = Frame(nb)
page4 = Frame(nb)
nb.add(page0, text="0")
nb.add(page1, text="1")
nb.add(page2, text="2")
nb.add(page3, text="3")
nb.add(page4, text="4")

Only tabs 0 and 1 show.  Add a space before or after the number and 2 & 3 show.  Add 6 spaces after 4 and '4 ' shows.  Appears to work OK with 3 chars, with first and third non-blank.

I presume this is a ttk bug.  http://www.tcl.tk/man/tcl8.6/TkCmd/ttk_notebook.htm#M14 just says 'a string'.  I plan to close this as 3rd party in a few days, but I wanted to document the defacto spec here on the tracker.