min_num on formsets should not just be added to extra, again

[typonaut]

I have read #22628 and #27679.

I can see that the documentation says that if you have min_num = 1 and extra = 1 then you will get two blank inline forms in a new form.

I am flagging this as a new feature, although I think it is really a bug.

When using forms.models.inlineformset_factory min_num and extra input parameters are available. Logically min_num is really a validation parameter where extra is a utility/UI parameter. I believe that given this logic, with a new form and min_num = 1 and extra = 1 then there should only be one inline form displayed, rather than two as currently happens.

I would propose to add an additional input parameter to control this bahaviour and account for any backward compatibility issues: min_num_minimise as a boolean.

forms.models.inlineformset_factory(… 
                  extra=1,
                  min_num=1,
                  min_num_minimise=True
                  )

This would render one inline form on a new form, where:

forms.models.inlineformset_factory(…
                  extra=1,
                  min_num=1,
                  min_num_minimise=False,
                  )

Would maintain the current behaviour – with the default being min_num_minimise=False.

I believe that the current implementation is more than a little confusing from a UI perspective. While it is possible to code around this implementation and test for at least min_num forms, that is quite a lot of work and more prone to break with updates.

In addition, there is a bug in behaviour: if min_num is greater than zero, a form action that deletes all existing inline forms is valid, which should not be the case.