App Router: Parent layout + parent loading rendered together instead of child loading — is this expected behavior?

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Summary

The Next.js App Router is exhibiting unexpected behavior with nested layouts and loading components. When navigating to a dynamic route, the parent layout remains rendered, but instead of showing the child loading component, the parent loading component is rendered on top, resulting in both the parent layout and parent loading being displayed simultaneously.

Root Cause

The root cause of this issue is due to the following factors:

  • Inconsistent rendering of the child loading component
  • Incorrect prioritization of the parent loading component over the child loading component
  • Dynamic route transitions causing unpredictable behavior

Why This Happens in Real Systems

This behavior occurs in real systems due to the complexities of nested route structures and asynchronous loading of components. The App Router may prioritize the parent loading component over the child loading component, leading to unexpected rendering behavior.

Real-World Impact

The real-world impact of this issue includes:

  • Poor user experience due to inconsistent and unpredictable loading behavior
  • Difficulty in debugging and identifying the root cause of the issue
  • Increased development time spent on troubleshooting and resolving the issue

Example or Code

// layout.tsx
import { useState, useEffect } from 'react';

const Layout = () => {
  const [loading, setLoading] = useState(false);

  useEffect(() => {
    // simulate loading state
    setLoading(true);
    setTimeout(() => {
      setLoading(false);
    }, 2000);
  }, []);

  return (
    

      {loading ? 
Parent loading...
 : 
Parent layout
}
    

  );
};

export default Layout;
// loading.tsx
const Loading = () => {
  return 
Child loading...
;
};

export default Loading;

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Implementing a consistent loading state management system
  • Prioritizing the child loading component over the parent loading component
  • Using debugging tools to identify and resolve the root cause of the issue
  • Testing and verifying the fix to ensure consistent behavior

Why Juniors Miss It

Junior engineers may miss this issue due to:

  • Lack of experience with complex nested route structures and asynchronous loading
  • Insufficient understanding of the App Router’s prioritization and rendering behavior
  • Inadequate testing and debugging techniques to identify and resolve the issue