After running `start` on an indeterminate Progressbar, it animates to one side, goes back to the other and then right before it reaches the end it disappears. I've attached a sample script below and a screen recording. This is running on macOS 11.3 and python 3.9.6 with the tkinter version that is packaged with the 3.9.6 installer. 


```
from tkinter import *
from tkinter.ttk import *
import time

window = Tk()
window.title('Test')
window.geometry('400x250+1000+300')


pb = Progressbar(window, orient=HORIZONTAL, length=100, mode='indeterminate')
pb.pack(expand=True)

Button(window, text='Start', command=pb.start).pack()

window.mainloop()
```

I can reproduce this behavior when using the current default 3.9.x macOS installer which uses Tk 8.6.8. It appears to work correctly if you instead use the alternate 3.9.x macOS universal2 installer which bundles Tk 8.6.11. The universal2 variant will be the default for Python 3.10 and may likely become the default for a future 3.9.x release. See if it solves the problem for you.

https://www.python.org/downloads/release/python-396/

It looks like that has done the trick. Is there an expected time when there won't be different installers?

It sounds like the fix is a configuration change already included in the next version, so ... I think that counts as a fix.

Phil, there are still some as yet unresolved differences between using Tk 8.6.8 and 8.6.11, both positive and negative, and there are a few differences between using the two variants under all conditions that we want to resolve. So, at the moment, we will continue to supply the two variants for 3.9.x bugfix releases with the traditional (non-universal2) variant remaining the default but that may change in the future.