Use of Spellchecker inside a designMode document in iframe

I have an extension where the user edits text in an iframe in an XUL
window. I set designMode to on in the iframe to allow editing of the
document.

If I open the XUL window with window.open, then I get the full set of
spellcheck facilities - misspelled words are underlined, I can right-
click on them to get corrections or do "Add to Dictionary" etc.

But if I open it with window.openDialog, then I only get underlined
misspelled words, none of the right-click facilities are available.

I'd like to use openDialog but have all of the spellcheck
functionality. Does anyone have any hints on this?