The "ApplyPlatformNavigation()" extension is defined in EPiServer.Shell.Navigation.MenuHelper. The class library documentation hasnt caught up to this yet.

@using EPiServer.Shell.Navigation

You add the navigation menu with (located in EPiServer.Shell.Navigation):

@Html.Raw(Html.CreatePlatformNavigationMenu())

And add the class for correct padding/margin etc with (located in EPiServer.Shell.Navigation):

@Html.Raw(Html.ApplyPlatformNavigation())

And put together:

    @Html.Raw(Html.ShellInitializationScript())
    @Html.Raw(Html.CreatePlatformNavigationMenu())
    <div id="epi-application" @Html.Raw(Html.ApplyPlatformNavigation())>
        @RenderBody()
    </div>

You could also try replacing ApplyPlatformNavigation with ApplyFullscreenPlatformNavigation located in EPiServer.Shell.Navigation.Internal.

A-ha, yes! Thank you. My actual view had the incorrect method name. I was using

<div @Html.Raw(Html.ApplyFullscreenPlatformNavigation())>
    <!-- Content -->
</div>

Which came from one of the code samples in https://world.episerver.com/blogs/ryan-bare/dates/2019/7/new-cms-ui-navigation/, but the descriptive text above that code sample had the correct method name: @Html.Raw(Html.ApplyPlatformNavigation()).

Thanks for clearing that up!

How do you replace the ShellMenu?

We use:
 
<%@ Register TagPrefix="sc" Assembly="EPiServer.Shell" Namespace="EPiServer.Shell.Web.UI.WebControls" %>

<sc:ShellMenu ID="ShellMenu2" runat="server" SelectionPath="/global/mycompany/admin" />