What problem does this address?

The Navigation block Submenus always display as dropdowns (unless the Overlay option is selected for all screen sizes). This layout is optimised for horizontal navs, but it doesn't work great on vertical ones. This is a fairly common website pattern:

But ideally, instead of the submenu displaying in a dropdown, it should either be visible at all times, or expand/collapse in an accordion pattern.

What is your proposed solution?

I tried solving this problem with some hacky CSS to always display the submenus:

.wp-block-navigation.is-vertical {
    .wp-block-navigation-submenu {
        display: block !important;
    }
    .wp-block-navigation__submenu-icon {
        display: none !important;
    }
    .wp-block-navigation__submenu-container {
       /* reset submenu dropdown styles */
        visibility: initial !important;
        opacity: initial !important;
        position: initial !important;
        height: initial !important;
        width: initial !important;
    }
}

It doesn't take a huge amount of restyling 😄

But a better solution for the block might be to add another configuration option to disable the submenu dropdowns (and maybe make that the default if the nav has a vertical orientation?)

We could also consider adding an accordion option.