Why don't the links in the slide menu area work when the slide menu is removed?

Question

css

Problem on the page www.ice-nut.ru/5/czech2.htm
When narrowing the screen in such a way that the left slide menu slides in, the links stop working (as I understand those located in the area where the extended slide menu would be). For example, page buttons 1, 2, ...6. Buttons that do not fall into the slide menu zone, for example 8, 9, 10 work correctly, links at the top and bottom of the page also work correctly. How to make links work while maintaining the correct operation of the menu and other functionality?
(something is visible with layers and positioning)