Looks like it's a problem with CKEditor, the rich-text-editor, that affects/has been affecting newer versions of Safari. This site uses an older version of CKEditor, so if alexp were ever to upgrade the RTE again, we might find that the problem has been resolved since then; however, even that's not a guarantee.
Barring trying perhaps another browser or something else on your end, not much can be done about it immediately, I'm afraid.