If IDLE were to recalculate the end from start, it would have to more or less parse forward to find the end of the statement.  This is why I considered it a speculative wish.

> would `end_lineno=None` make any sense?  
Only as a kludge if a default were required because of the signature position.

Point 3: The existing doc implies but does not exactly state that the _Object signature are private, so that we could break any possibly existing (but possibly not, and discouraged) use.  I think back-compatibility is discussed in the devguide.

Posts do not have a link until distributed, and then you can find it on mail.python.org.  The title is sufficient to do that.