Main menu submenu is to long

give those menus an overflow : auto and a concrete height (f.e. 60vh means 60% of screenheight) or as code shown a calculated height
I have this sometimes to do on mega-menus that have a second, third row i gave in this case to the mega-menu:

#header .avia_mega_div {
    overflow: auto !important;
    max-height: calc(100vh - 80px);
}

i can not see your site – i’m participant too – so i can not give you the code which works for you

supposed it is working:

.avia-menu .sub-menu {
    overflow: auto !important;
    max-height: calc(100vh - 80px);
}

or test this:

#top #header  ul:first-child >li > ul  {
    overflow: auto !important;
    max-height: calc(100vh - 80px);
}

the 80 px depends on your header height