Mega menu is not always showing for the first item

[Jura]

The issue is not always showing, and it is apparently specific to Webkit browsers (Chrome & Edge), Firefox works fine.

When user is accessing mega menu for the first time after load, some times background is not fully shown:

Replicable at https://www.undp.org and https://design.undp.org/?path=/story/templates-global-homepage--global-homepage-story, but not at https://design.undp.org/iframe?globals=&args=&id=templates-global-homepage--global-homepage-story&viewMode=story

[Jura]

If viewport is really small (in 1024-1100) range first mega menu item is also blinking without fully opening dropdown