How to fix a bug in the menu when hovering over a dropdown list?

Question

css

Good day.
I can not understand why it was formed and how to fix an unpleasant bug. Website
available . When you hover over the top item in the left menu, a submenu appears. And everything seems to be cool while the menu is above the slider, but if the page is scrolled down a little, so that the submenu drops out above the introductory block, then a jamb pops out. Items in the main block that have the property position: relative; for some reason they are in a higher priority than the submenus and are shown on top. Please tell me why this is happening and how to deal with it.

Answer 1

.left_aside set this block z-index: 9 for example and the issue will be resolved.