It looks like when there is a long word/URL in an edit summary and it does not fit the popup, sometimes the scrollbar is missing.
E.g. on https://de.wikipedia.org/w/index.php?diff=121648836&oldid=99344045&title=Wikipedia%3AHauptseite&type=revision when hovering the newer revision, there is a scrollbar but when you scroll forward with an arrow, and hover this revision again, there is no scrollbar and contents of the popup are "clipped".