Heh, reminds me of a bug I ran into a couple of weeks ago. Users on Wikipedia were complaining that typing into the editor <textarea> on certain pages was sluggish. I eventually narrowed it down to the presence or absence of an invisible left-to-right or right-to-left mark control characters elsewhere in the page. See https://bugzilla.mozilla.org/show_bug.cgi?id=1296050 for the gory details.