The edit summary window should be the next tab stop after the editing window; e.g. so the user presses TAB once to jump to the edit summary window after finishing their comment. Overall this tool is a very welcome improvement; thank you.
Topic on Talk:Talk pages project/Replying/Flow
I've been thinking about this. The edit summary field is collapsed/hidden for most editors, and mostly unused by editors. (A few people use it a lot, but most people use it never or rarely.) It would be undesirable to "lose" the cursor in an invisible/collapsed edit summary window, but I'm not sure if that would be a problem. The tab system might be able to skip it/detect when the edit summary is open.