Here is what I've come up with, attempting to get FEE to work with IE6, 7, and 8, Firefox, Chrome, Safari, and Opera (on Windows; no idea about Mac or Linux browsers).
1) IE 6, 7, 8 seem to require the following surgery on nicedit.js: in lines 323 and 419 (of the nicedit.dev.js file), I changed
(parseInt(e.getStyle('width')) || e.clientWidth)
to
(e.clientWidth || parseInt(e.getStyle('width')))
I found this suggestion on the nicedit forum -- others have had the same problem with nicEdit on IE, and not just within WordPress.
(Note that this "||" is the rather unusual "assignment or", not a logical or: If the first value exists/is "true", that value is returned, otherwise the second value is.)
This fix seems quite necessary for all IE's -- I see the problem, and the success of this fix, on several quite different sites using a number of different themes, including classic and default. Fortunately, I haven't seen any negative effect due to it.
2) Because width is not inherited, it may be necessary that the enclosing div be given a width explicitly in the css. But not always -- for example, not in the default theme.
3) And several browsers (IE and Opera are the fussiest) don't like widths in % or auto or inherit. Only absolute units (px, em ...) seem to work for all browsers.
This is a flakey problem. It is real, but not universal. It is not a simple WordPress plugin interaction, since it occurs even with all other plugins deactivated (though some may have left traces in database's wp_options -- perhaps that is where the problem is, and why it sometimes seems to appear/disappear spontaneously). And it occurs outside WordPress (see the nicedit forum), so it's not just a plugin interaction.
Each of the "cures" mentioned above is quite repeatable within a specific situation (site, theme, browser) -- that is, if I remove the fix, the problem returns, and goes away again when re-fixed. But, as I said, they are not all necessary for all setups -- except perhaps for the first fix.
Feedback?
Kim Kirkpatrick