When displaying a preview, an empty line is appended to the edited wikitext in EditPage::getPreviewText().
I don’t know what was the point of this, however, it is wrong: When the wikitext ends with categories or interwiki, this empty line is trimmed, so that it has no effect at all. In other cases, the preview looks different to the final result, which is against the definition of preview.
If some margin above the edit window/toolbar is intended, it should be displayed consistently (no matter if the article contains categories or not), and as that margin would only be a matter of design, it should be done with CSS, not with that <p><br /></p>.
Version: unspecified
Severity: minor