Document #[end_layout] attribute and layout scoping behavior

[ThomasSteinbach]

Problem

The router documentation does not explain the stateful scoping behavior of the #[layout()] attribute, nor does it mention the #[end_layout] attribute needed to close layout scopes.

This leads to a common bug where developers accidentally create nested/duplicate layouts when trying to apply the same layout to multiple routes.

Current Documentation Gap

The layouts documentation shows only single-route examples:

#[layout(Wrapper)]
    #[route("/")]
    Index {},

When developers want multiple routes to share the same layout, they naturally assume they need to add #[layout()] to each route:

// ❌ Creates NESTED layouts (2 Wrapper components rendered)
#[layout(Wrapper)]
#[route("/")]
Home {},

#[layout(Wrapper)]  // This NESTS inside the first layout!
#[route("/tokens")]
Tokens {},

This results in duplicate UI elements (e.g., 2 navigation bars, 2 headers) with no compile-time warning.

Root Cause

The #[layout()] attribute creates a persistent, stateful scope that remains active for all subsequent routes until explicitly closed with #[end_layout] (similar to how #[nest()] works with #[end_nest]).

What Should Be Documented

1. Layout scoping is stateful - Once opened, it affects all subsequent routes
2. The #[end_layout] attribute exists - Explicitly closes a layout scope
3. Proper indentation convention - Visual hierarchy helps prevent mistakes
4. Multiple routes under one layout - Show the correct pattern:

#[layout(Wrapper)]
    #[route("/")]
    Home {},
    
    #[route("/tokens")]
    Tokens {},
#[end_layout]

5. Warning about duplicate layouts - Explain what happens if scopes aren't closed
6. Debugging technique - Mention cargo expand to verify nesting levels

Real-World Impact

This bit us in production - our /tokens page rendered with 2 complete layouts (duplicate navigation, duplicate sidebars) while / rendered correctly. Debugging required:

• Inspecting generated HTML with curl
• Using cargo expand to examine macro-generated code
• Reading Dioxus router source code

The fix was simple (#[end_layout]), but discovering it was not.

Comparison with Nested Routes

The nested routes documentation does explain this pattern for #[nest()]:

To finish a nest, we use the #[end_nest] attribute or the end of the enum.

Layouts should have equivalent documentation for #[end_layout].

Proposed Solution

Add a new section to the layouts documentation showing:

• How to apply one layout to multiple routes
• The #[end_layout] attribute
• Warning about accidental nesting
• Example of the wrong pattern vs correct pattern
• Optional: debugging tips

References

• Tutorial code that works (but doesn't explain why): https://dioxuslabs.com/learn/0.7/tutorial/routing#adding-a-favorites-route
• Nested routes docs (good model to follow): https://dioxuslabs.com/learn/0.7/essentials/router/routes#nested-routes

[ThomasSteinbach]

Fixed with #628