From f7bb0201d73ce0acf8e9cf17e2f487f8b3195c54 Mon Sep 17 00:00:00 2001
From: Manuel Schiller <manuel.schiller@caligano.de>
Date: Sat, 24 Jan 2026 21:20:22 +0100
Subject: [PATCH] fix: streaming race condition

---
 packages/router-core/src/ssr/transformStreamWithRouter.ts | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/packages/router-core/src/ssr/transformStreamWithRouter.ts b/packages/router-core/src/ssr/transformStreamWithRouter.ts
index ef893bfa9e2..f7d856d7622 100644
--- a/packages/router-core/src/ssr/transformStreamWithRouter.ts
+++ b/packages/router-core/src/ssr/transformStreamWithRouter.ts
@@ -271,11 +271,13 @@ export function transformStreamWithRouter(
       const html = router.serverSsr?.takeBufferedHtml()
       if (!html) return
 
-      if (isAppRendering) {
-        // Buffer for insertion at next valid position
+      if (isAppRendering || leftover) {
+        // Buffer when app is still rendering OR when there's leftover content
+        // that hasn't been flushed yet. This prevents race conditions where
+        // injected HTML appears before buffered app content
         pendingRouterHtmlParts.push(html)
       } else {
-        // App is done rendering, write directly to output
+        // App done rendering and no leftover - safe to write directly for better streaming
         safeEnqueue(html)
       }
     })