I just made an edit and saved it without giving it an edit summary. The default summary was
added 24 characters in body
I made another edit to the same post within 5 minutes, this time changing the automatic edit summary, however, when I viewed the edit, the original automatic summary remained.
I suggest that we be allowed to edit the summary while we're within 5 minutes of posting the edit.
It has been suggested that users can make two edits to change the summary: the first by making a superfluous change, the second by undoing the change and changing the summary at the same time. This is counterintuitive and surprising behaviour. A user only has 5 minutes to change an edit before it becomes a new edit, and in that period they should be able to do anything they could have done on the first edit.
This feature was first suggested in early 2011 and was very popular, yet was never done. If the reason it hasn't been implemented is to avoid trivial edits, then the workaround (which didn't work for me) simply doubles the number of trivial edits needed to get the job done.