Just thought I'd add this for anyone else with the same problem.
I've found that combining CSS files with xFPC breaks the media screen size queries on reload for responsive layouts.
I thought this may be down to the combined css link not including media="screen", so added that to line 281 of the xFPC plugin and it fixed the issue.
Replacement line 281 of plugins/xFPC:
$cssCombinedFile = '<link rel="stylesheet" type="text/css" href="'.$modx->getOption('assets_url').'components/xfpc/cache/'.$cacheHash.'.css" media="screen" />';
Also if you're using fall back CSS for IE8 make sure to add ie8.css (or whatever your filename is) to the excluded CSS files in the system settings.
[ed. note: absent42 last edited this post 13 years, 7 months ago.]
