[issue41176] revise Tkinter mainloop dispatching flag behavior

[E. Paine]

E. Paine <paineelisha at gmail.com> added the comment:

I agree it would be helpful to expose an explicit way of telling if the mainloop was running but am not sure about removing `WaitForMainloop` as it could very easily break existing programs.

If a program executes a tkinter method in a thread before the mainloop is executed, the method will wait because of the call to `WaitForMainloop`. In the example script this is done deliberately to demonstrate the behaviour but could be done accidentally if the main thread has to do something else before the mainloop (and after the thread has been created).

I think the changes (whatever is concluded we should do) would be considered an 'enhancement', which would not be backported to 3.9 and before (I believe 'behaviour' is generally used for logic errors).

I am very willing to help review a PR, however the people you really need to convince are Serhiy and/or Guilherme (I have added them to the nosy).

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nosy: +epaine, gpolo, serhiy.storchaka
versions:  -Python 3.5, Python 3.6, Python 3.7, Python 3.8, Python 3.9
Added file: https://bugs.python.org/file49283/waitmainloop.py

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