Navigation: Submenus not hidden on older browsers

[ryelle]

Originally reported to the WordPress.org header repo, WordPress/wporg-mu-plugins#159. The navigation menu dropdowns use :where to hide the submenus, which does not work on Chrome 87 & below (source: Can I use). Chrome 88 was released in Jan 2021.

You can see the issue in the following screenshots, on both the custom code for WordPress.org & the demo site for Twenty Twenty-Two (the "Sub-page" is visible like a top-level item).

Chrome 87	Firefox 97

While this does match with the supported browserslist, there was some discussion about whether this list should also apply to the frontend of sites (discussion starts here).

@joyously The reason I brought it up is because there is a list of supported browsers for the WP product. But it seems to be the supported list for the .org site (and apparently all sites created by the WP product).
Usually this list only affects the admin. Now it's starting to affect the front end of WP sites.

[iandunn]

There's a new discussion in #39621 about the larger problem

[jasmussen]

#42964 aims to fix this issue, and links out to 2 potential addons.

[jasmussen]

This should be fixed by #42967.