const router = createRouter({                                                                                                                   
  defaultViewTransition: {                                                                                                                      
    types: ({ fromLocation, toLocation }) => {                                                                                                  
      // This always runs, even if the browser already animated                                                                                 
      return [`slide-${direction}`];                                                                                                            
    },                                                                                                                                          
  },                                                                                                                                            
});                                                                                                                                             

  // Internal TanStack Router logic (pseudocode)                                                                                                  
  if (!event.hasUAVisualTransition && defaultViewTransition) {                                                                                    
    document.startViewTransition(() => { /* ... */ });                                                                                            
  } else {                                                                                                                                        
    // Proceed without view transition                                                                                                            
  }