Adam, there is no DNN nav menu - nav is a wrapper for the menu provider specified in web.config. The issue might be caused by misinterpretation of browser caps - but in this case it would be a constant issue, you'd experience on visits of all DNN sites using it. Another cause (I suspect to be more likely) is a caching issue, if you visit a page just after a downlevel spider accessed it.

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Cheers from Germany,
Sebastian Leupold

   

Speed up your DNN Websites with TurboDNN

I found a workaround for the browser itself and something that hopefully someone else can build off of.
I have this user agent switcher for Firefox and the menus appear normally if I switch the user agent string. After I found that out I tried to discover what aspect was throwing it off and I'm not dead set on what it was but I have a likely suspect and a work around.
My default user agent string is "Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.8) Gecko/20100723 Ubuntu/10.04 (lucid) Firefox/3.6.8" I tried taking out bits and pieces and found online that most examples had the Firefox portion right after the Gecko portion like so "Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.8) Gecko/20100723 Firefox/3.6.8 Ubuntu/10.04 (lucid)" which fixes it. I've tried taking out the (lucid) alone and it doesn't help. Something with either DNN or ASP.net is messing up while parsing the user agent string I suppose.

This worked for me and I plan to look into more but if other could test with their own system that would help, you can message me your before and after to help fix this out. There are plenty of sites like http://user-agent-string.info/parse that will tell you your user agent string then goto into about:config and make/edit general.useragent.override and place the modified user agent string there.

*Note, after you change your user agent string by any method and reload the page the menus may disappear, just do a Ctrl+F5 to reload the page.