fix: SectionOutlet in Navigation area throws "There is already a subscriber" when resizing across the mobile breakpoint

[Asttear]

🐛 Bug Report

FluentLayout.NavigationDeferredLoading="true" + a SectionOutlet in the LayoutArea.Navigation item crashes with There is already a subscriber to the content with the given section ID when the window is resized across the 768px mobile breakpoint.

💻 Repro or Code Sample

MainLayout.razor (drop into any FluentUI v5 Blazor app — or simply add a SectionOutlet to the LayoutArea.Navigation item of the FluentUI demo's DemoMainLayout, which already uses NavigationDeferredLoading="true"):

@inherits LayoutComponentBase

<FluentLayout NavigationDeferredLoading="true">
    <FluentLayoutItem Area="@LayoutArea.Header">
        <FluentStack VerticalAlignment="VerticalAlignment.Center">
            <FluentLayoutHamburger Display="@HamburgerDisplay.DesktopMobile" />
            <SectionOutlet SectionName="topbar" />
        </FluentStack>
    </FluentLayoutItem>

    <FluentLayoutItem Area="@LayoutArea.Navigation" Width="300px">
        <SectionOutlet SectionName="sidebar" />
    </FluentLayoutItem>

    <FluentLayoutItem Area="@LayoutArea.Content">@Body</FluentLayoutItem>
</FluentLayout>

A page provides the content:

<SectionContent SectionName="sidebar">
    <div>Some sidebar content</div>
</SectionContent>

Steps to reproduce:

1. Run the app at a desktop window width (e.g. 1280px).
2. Resize the window below the 768px mobile breakpoint — or back across it.

🤔 Expected Behavior

The layout adapts to the mobile viewport without error. The inline Navigation item and the hamburger drawer should render mutually exclusively across the breakpoint change.

😯 Current Behavior

Resizing throws an unhandled exception (crashes the app):

System.InvalidOperationException: There is already a subscriber to the content with the given section ID 'System.Object'.
   at Microsoft.AspNetCore.Components.Sections.SectionRegistry.Subscribe(Object identifier, SectionOutlet subscriber)
   at Microsoft.AspNetCore.Components.Sections.SectionOutlet.Microsoft.AspNetCore.Components.IComponent.SetParametersAsync(ParameterView parameters)
   at Microsoft.AspNetCore.Components.Rendering.ComponentState.SupplyCombinedParameters(...)

The section ID string shown depends on the identifier used — SectionName shows the name; a SectionId object shows its ToString().

💁 Possible Solution

FluentLayoutHamburger.NavigationContent defaults to the Navigation area's ChildContent:

NavigationContent => ChildContent ?? LayoutContainer?.Areas.Find(i => i.Area == LayoutArea.Navigation)?.ChildContent

On a breakpoint flip, FluentLayout.FluentLayout_MediaChangedAsync refreshes hamburgers before Navigation-area items, as two separate render batches:

foreach (var item in Hamburgers) await item.RefreshAsync();
foreach (var item in Areas.Where(i => i.Area == LayoutArea.Navigation)) await item.RefreshAsync();

With NavigationDeferredLoading="true", crossing the breakpoint flips RenderDrawer() (hamburger) and RenderThisArea() (nav item) in opposite directions. Because the hamburger renders first, the drawer's mirrored NavigationContent instantiates a second SectionOutlet with the same section ID while the nav item's outlet is still subscribed → SectionRegistry.Subscribe throws.

Suggested fix: refresh the Navigation-area items before the hamburgers in FluentLayout_MediaChangedAsync (so the old outlet unsubscribes before the drawer's mirror subscribes), or otherwise make the inline-nav / drawer-in-drawer rendering mutually exclusive within a single render batch. Happy to open a PR if the ordering approach is acceptable.

🔦 Context

Desktop tool whose page feeds a sidebar through SectionOutlet/SectionContent. The window is resizable; shrinking it across the 768px breakpoint (or back) crashes the whole app, making the layout unusable on smaller windows.

🌍 Your Environment

• OS & Device: Windows 11 on PC
• Browser: Edge WebView2 (Blazor WebView / Hybrid host)
• .NET and Fluent UI Blazor library Version: .NET 10.0, Microsoft.FluentUI.AspNetCore.Components 5.0.0-rc.4-26180.1