I had exactly the same problem
HTMLeditor removes the closing tag of the iframe before preview and I think iframe's don't work without closing tags and caused the display problem
edit: editors/htmlarea/htmlarea.js
find
PHP Code:
HTMLArea.needsClosingTag = function(el) {
var closingTags = " head script style div span tr td tbody table em strong font a title ";
return (closingTags.indexOf(" " + el.tagName.toLowerCase() + " ") != -1);
};
(line 3893)
add iframe
e.g.:
PHP Code:
HTMLArea.needsClosingTag = function(el) {
var closingTags = " iframe head script style div span tr td tbody table em strong font a title ";
return (closingTags.indexOf(" " + el.tagName.toLowerCase() + " ") != -1);
};