Umami main menu focus style is missing the top outline in Firefox

Thanks for picking this up. Sorry, this may be a confusing one, because it's is a follow-up to #2983568: Audit and improve focus styles across the Umami theme for logged out users, which hasn't been committed yet. This isn't our usual way of working though - in this case the previous issue is quite epic, an this was deferred so that it wouldn't hold up all the wins in the main issue. It's probably best to leave this issue until after that one is committed. I've marked this postponed, and I'll set it back to active when at the right time.

Is there an accessibility setting switched on that activates the dotted line borders?

That depends on your operating system. MacOS has a setting (a stupid one, IMO) to enable tabbing to links. It's somewhere in the System Settings application. Firefox respects that setting.

This issue can be active again, since #2983568: Audit and improve focus styles across the Umami theme for logged out users is fixed.

The problem still exists as of the current 8.8.x. See this screenshot:

The problem seems to be overflow on the list wrapper. If I use browser zoom to reduce to 90% scale, the top outline is visible:

Digging around in the CSS, I found these:

core/profiles/demo_umami/themes/umami/css/components/navigation/menu-main/menu-main.css

.menu-main {
  overflow: hidden;
  // ...
}

@media screen and (min-width: 48em) {
  .menu-main {
    // ...
    overflow: auto;
    // ...
  }
  // ...
}

Disabling both of these overflow declarations makes the top outline visible. What are these used for, and is there anything that will break if we remove them?