### Background
Vector checkbox hack implementations use `visibility: hidden` to hide the elements when the checkbox is not checked. This removes these elements from the accessibility tree, rendering them inaccessible to screenreader users.
Because the main menu and collapsed TOC have landmarks in the hidden content itself, this means the landmark disappears when the checkbox is not checked. This is mentioned with the main menu in T311294.
This is generally fine for the user menu and most dropdowns, as they are generally entirely contained in a landmark, and hiding the menu contents matches the experience for visual users
### Developer notes
The simplest solution is to use `.ui-helper-hidden-accessible`or equivalent CSS on the main menu and collapsed TOC to visually hide the contents. However this should be a temporary solution, as it would make the TOC and main menu always available to screenreaders even when the menu is closed.
We could also update the HTML/CSS to ensure that the nav element is not hidden when the menu is closed. However, this alone won't help the user experience much because the trigger is not located in the landmark. An ideal solution would ensure the menu trigger is inside the nav landmark, which could be possible for the main menu when we move it to a dropdown menu.
Vector dropdown menus are generally entirely contained in a landmark so they don't need to be changed, thought it's probably worth investigating at some point if we should consider avoid using visibility: hidden entirely, as maybe screenreader users don't want to hide those elements in general.
### AC
[] Main menu and "Site navigation" landmark is still accessible to screenreaders when the menu is closed
[] TOC and "Contents navigation" landmark is still accessible to screenreaders when the menu is closed