With the parameter summary you can prefill the edit summary field. Example:
https://en.wikipedia.org/w/index.php?title=Foobar&action=edit&summary=Summary
This also works for editing old versions:
https://en.wikipedia.org/w/index.php?title=Foobar&action=edit&oldid=454714833&summary=Summary
But it doesn't work when you undo a revision:
https://en.wikipedia.org/w/index.php?title=Foobar&action=edit&undoafter=454714833&undo=454714890&summary=Summary
If the parameter is specified it should even override the default summary for undoing.
Version: 1.18.x
Severity: enhancement