Since your menu is on another z-index, would it be possible to use visibility
instead?
Here's a DEMO
You need to change these two things
header nav > ul > li > ul {
/*display:block;*/
visibility:hidden;
}
header nav > ul > li.drop:hover > ul {
/*display:block;*/
visibility:visible;
}
As to why IE10 messes up the display:block;
style in this case I'll leave to someone else to explain.