From ac09f76523424b140f498653c2109741b6d18f52 Mon Sep 17 00:00:00 2001
From: Domenic Denicola <d@domenic.me>
Date: Fri, 23 May 2025 15:26:36 +0900
Subject: [PATCH] Delay background sync while prerendering

Closes #354.

The only relevant background sync API accessible from Windows is serviceWorkerRegistration.sync.register(). This registers a sync to happen later when the user is online.

At first I thought allowing this to happen while prerendering might be OK. However, the method explicitly throws an exception if it's used when there are no top-level/auxiliary clients. https://developer.chrome.com/blog/background-sync/ seems to imply this is an abuse prevention measure: "You can only register for a sync event when the user has a window open to the site."

So, we decide to delay any background sync registrations while prerendering.
---
 prerendering.bs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/prerendering.bs b/prerendering.bs
index f1a4c9a..52db8d6 100644
--- a/prerendering.bs
+++ b/prerendering.bs
@@ -945,6 +945,10 @@ Add {{[DelayWhilePrerendering]}} to {{PushManager/subscribe()}}.
 
 Add {{[DelayWhilePrerendering]}} to {{BackgroundFetchManager/fetch()}}.
 
+<h4 id="background-sync-patch">Background Sync</h4>
+
+Add {{[DelayWhilePrerendering]}} to {{SyncManager/register()}}.
+
 <h4 id="persist-patch">Storage API</h4>
 
 Add {{[DelayWhilePrerendering]}} to {{StorageManager/persist()}}.