fix: handle undefined children in isSameRoute for sibling layouts

[scott-memco]

Summary

Fixes RSC navigation crashing with "can't access property 'every', newRoute.children is undefined" when navigating between routes in sibling layouts.

Root Cause

In isSameRoute() (packages/react-router/lib/router/router.ts), when comparing pathless layout routes during patchRoutesOnNavigation:

1. The server returns route patches as flat objects without a children property
2. The "both empty" check (lines 5374-5379) only returns true when both routes lack children
3. When newRoute.children is undefined but existingRoute.children has items, the condition evaluates to false and falls through to newRoute.children!.every(...) which crashes

Fix

Add an explicit guard after the joint empty-children check: if either route has children but the other doesn't, they are definitively different routes. This prevents the null dereference and is semantically correct — two pathless layouts with mismatched children structures are distinct routes.

// New guard: asymmetric children means different routes
if (!newRoute.children?.length || !existingRoute.children?.length) {
  return false;
}

This also removes the non-null assertion (!) on newRoute.children since the guard guarantees both sides have children at that point.

Reproduction

From the issue's minimal repro:

// This crashes when navigating from route-a to route-b:
layout("layouts/layout-a.tsx", [route("/route-a", "routes/route-a.tsx")]),
layout("layouts/layout-b.tsx", [route("/route-b", "routes/route-b.tsx")]),

Fixes #14777

Made with Cursor

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