Because the <ul> elements with visibility: hidden still take up space. See screenshotpic.twitter.com/eBrHsJzZyW
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Yes, @pcwalton solution works! From what I (quickly) tested, just changing the "visibility: hidden" part to "display: none" fixes the problem without causing any side effects, because you're already setting "display: block" on hover.
darn, I missed the actual reason :p
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