NextJS + MUI app does not follow the mode set by the Operating system

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Summary

A Next.js application using Material UI (MUI) fails to correctly apply the operating system’s dark mode preference when the browser’s theme is set to “Device.” Instead of respecting the OS setting, the application renders exclusively in light mode. The root cause is a mismatch between the client-side hydration logic in MUI’s InitColorSchemeScript and the server-side rendering context, where the operating system’s system preference is unavailable.

Root Cause

The failure stems from how the InitColorSchemeScript component handles the defaultMode="system" prop during server-side rendering (SSR) and subsequent client-side hydration.

  • Server-Side Ignorance: When Next.js pre-renders the page on the server (SSR), it executes the layout code. The server environment does not have access to the client’s Operating System preference. When defaultMode="system" is encountered, MUI defaults to the light mode to prevent a flash of unstyled content or potential hydration errors.
  • Incorrect Script Generation: The InitColorSchemeScript injects a script tag to determine the initial color scheme. If the specific defaultColorScheme or strict logic isn’t enforced, the generated script may default to light when it cannot resolve a specific value, or it may not correctly detect the OS preference immediately upon browser execution.
  • Hydration Mismatch: If the server renders Light (the safe default) and the client script takes a moment to detect the OS Dark mode, React encounters a hydration mismatch. To resolve this mismatch, the hydration process often sticks to the server’s render (Light) or falls back to the script’s default, bypassing the OS check.

Why This Happens in Real Systems

  • Server vs. Client Disparity: The fundamental challenge of SSR is that the server is stateless regarding the client’s environment. It knows nothing about the user’s OS, local time, or device capabilities.
  • “Safe” Defaults: Libraries like MUI prioritize stability. “System” is an ambiguous state on the server. To avoid crashing or visual jank, the library implicitly defaults to “Light” for the initial HTTP response.
  • Timing Issues: The detection of OS preference (prefers-color-scheme) is a browser API. It is not available until the JavaScript runtime is active. If the UI paints before that detection completes, it will use the fallback.

Real-World Impact

  • Poor User Experience: Users with dark mode enabled on their OS (often to reduce eye strain or save battery) are blinded by a bright white interface on page load.
  • Inconsistent Branding: Users who have explicitly set their preference to “Device” (expecting OS sync) feel the app is broken because it ignores their system settings.
  • Cumulative Layout Shift (CLS): If the app eventually switches to dark mode after a delay (via a flash of unstyled content), it causes visual instability.

Example or Code

The issue is specifically with the InitColorSchemeScript component usage. Here is the corrected implementation ensuring the server does not guess, and the client strictly adheres to the system preference.

Corrected layout.jsx:

import { AppRouterCacheProvider } from '@mui/material-nextjs/v14-appRouter';
import { ThemeProvider, createTheme } from '@mui/material/styles';
import InitColorSchemeScript from '@mui/material/InitColorSchemeScript';
import theme from './theme';

export default function RootLayout(props) {
  return (
    
      
        {/* 1. Explicitly initialize the script with 'system' to match the provider */}
        
        
          {/* 2. Ensure defaultMode matches the script exactly */}
          
            {props.children}
          
        
      
    
  );
}

The Theme Definition (Ensure cssVarPrefix matches if used):

// theme.js
import { createTheme } from '@mui/material/styles';

const theme = createTheme({
  cssVarPrefix: 'my-app', // Optional, but must be consistent
  colorSchemes: {
    light: true,
    dark: true,
  },
  // ... other theme settings
});

export default theme;

How Senior Engineers Fix It

  • Strict Prop Synchronization: Senior engineers verify that defaultMode="system" is set on both the InitColorSchemeScript and the ThemeProvider. Any discrepancy causes fallback behavior.
  • Script Placement: They ensure the InitColorSchemeScript is placed as high as possible in the DOM (immediately after <body>) so that the attribute is set on the <html> or <body> tag before MUI’s CSS-in-JS generates styles.
  • Suppress Warning Correctly: They use suppressHydrationWarning on the <html> tag to allow the specific attribute changes MUI makes without triggering React errors, acknowledging that the server and client initial values might differ (Light vs System).
  • Theme Configuration: They verify that the createTheme object actually defines colorSchemes (or palette for older versions) for both light and dark, ensuring the “System” switch has valid targets.

Why Juniors Miss It

  • Misunderstanding SSR: Juniors often assume that logic placed in layout.js runs with full browser context. They do not realize the InitColorSchemeScript is a bridge to fix the very limitation of “Server does not know OS preference.”
  • Copy-Paste Errors: They copy configuration from documentation examples that might use defaultMode="light" or omit the script entirely, not realizing these are prerequisites for “System” mode to work.
  • Focus on UI, Not Configuration: The UI logic (Theme) is distinct from the initialization logic (Script). Juniors often fix the Theme but forget to initialize the Script that actually reads the OS preference.