.diff, .diff-hr and .diff-currentversion-title should not be part of #mw-content-text.
I only expect content inside #mw-content-text.
.diff, .diff-hr and .diff-currentversion-title is a part of the interface and should be a part of #bodyContent or #content.
In case someone did not understand the description, the following jQuery snippet produces the correct HTML structure and appearance:
$( '.diff, .diff-hr, .diff-currentversion-title' ).insertBefore( '#mw-content-text' );
Of course, that should be done server-side, not client-side, and with PHP, not JavaScript.