Basically, on a drop down sub menu if you set the width to 100% with padding, content overflows the background as you can see in this example http://notjusttalking.co.uk.cp-15.webhostbox.net/ - if you hover over 'publications' you will see that they have got really long nav points. (I appreciate that these nav points are pretty horrible but its dynamic and the client is in 'control' but even if they were shorter, the same would be happening.)
I've run into this. Set the submenu size to more than 100%.
I usually just keep making the submenu size larger until it fits the submenu button.
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