Convert the standard Blazor navigation menu to a collapsible icon menu

The issue seems to be more with Identity, the default .NET8 WebApp configuration for a interactive blazor server will set the render mode in the HeadOutlet as:

<HeadOutlet @rendermode="InteractiveServer" />

with the following route:

<Routes @rendermode="InteractiveServer" />

The issue you're seeing is because Identity is expecting the HttpContext to not be null.

AccountLayout.Razor

@if (HttpContext is null)
{
<p>Loading...</p>
}
else
{
@Body
}

@code {
[CascadingParameter]
private HttpContext? HttpContext { get; set; }

protected override void OnParametersSet()
{
if (HttpContext is null)
{
// If this code runs, we're currently rendering in interactive mode, so there is no HttpContext.
// The identity pages need to set cookies, so they require an HttpContext. To achieve this we
// must transition back from interactive mode to a server-rendered page.
NavigationManager.Refresh(forceReload: true);
}
}
}

This is raised here -https://github.com/dotnet/aspnetcore/pull/51134 A work around was provided in this issue - https://github.com/dotnet/aspnetcore/issues/51170 

You can update App.Razor with:

<HeadOutlet @rendermode="@RenderModeForPage" />

<Routes @rendermode="@RenderModeForPage" />

also including the following code block:

@code{
[CascadingParameter] HttpContext HttpContext { get; set; } = default!;

IComponentRenderMode? RenderModeForPage => HttpContext.Request.Path.StartsWithSegments("/Account")
? null
: RenderMode.InteractiveServer;
}

Which I've tested and seems to work, however the tests aren't extensive to say the least.