From 1a524345aac4066184ee97edaaec9ed4437b998a Mon Sep 17 00:00:00 2001
From: MartinBraquet <martin.braquet@gmail.com>
Date: Thu, 30 Jul 2026 01:00:58 +0200
Subject: [PATCH] Implement notification deduplication with `collapseKey` to
 prevent stacking across messages/conversations.

---
 backend/api/src/helpers/private-messages.ts    | 1 +
 backend/api/src/update-connection-interests.ts | 1 +
 backend/shared/src/mobile.ts                   | 7 +++++++
 web/lib/service/android-push.ts                | 4 +++-
 4 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/backend/api/src/helpers/private-messages.ts b/backend/api/src/helpers/private-messages.ts
index af8f7de0..85c6574c 100644
--- a/backend/api/src/helpers/private-messages.ts
+++ b/backend/api/src/helpers/private-messages.ts
@@ -186,6 +186,7 @@ const notifyOtherUserInChannelIfInactive = async (
     title: `${creator.name}`,
     body: textContent,
     url: `/messages/${channelId}`,
+    collapseKey: `channel-${channelId}`,
   }
   try {
     await sendWebNotifications(pg, receiverId, JSON.stringify(payload))
diff --git a/backend/api/src/update-connection-interests.ts b/backend/api/src/update-connection-interests.ts
index 098acc42..a8dba35c 100644
--- a/backend/api/src/update-connection-interests.ts
+++ b/backend/api/src/update-connection-interests.ts
@@ -78,6 +78,7 @@ export const updateConnectionInterests: APIHandler<'update-connection-interest'>
             },
           ),
           url: `/${currentUser.username}`,
+          collapseKey: `connection-${currentUser.id}`,
         }
         try {
           await sendWebNotifications(pg, targetUserId, JSON.stringify(payload))
diff --git a/backend/shared/src/mobile.ts b/backend/shared/src/mobile.ts
index 7f060e80..6dc8258d 100644
--- a/backend/shared/src/mobile.ts
+++ b/backend/shared/src/mobile.ts
@@ -89,6 +89,9 @@ interface PushPayload {
   url: string
   data?: Record<string, string>
   imageUrl?: string
+  // Notifications sharing a collapseKey replace each other instead of stacking, so a
+  // conversation only ever occupies one slot in the tray. See sendPushToToken.
+  collapseKey?: string
 }
 
 export async function sendPushToToken(
@@ -100,10 +103,14 @@ export async function sendPushToToken(
   const message: TokenMessage = {
     token,
     android: {
+      // FCM-side: if the device is offline, only the latest message of a conversation is delivered
+      collapseKey: payload.collapseKey,
       notification: {
         title: payload.title,
         body: payload.body,
         imageUrl: payload.imageUrl || undefined, // 👈 publicly accessible HTTPS URL
+        // Tray-side: a new notification replaces the previous one carrying the same tag
+        tag: payload.collapseKey,
       },
     },
     data: {
diff --git a/web/lib/service/android-push.ts b/web/lib/service/android-push.ts
index 1d52bcb7..1edb9151 100644
--- a/web/lib/service/android-push.ts
+++ b/web/lib/service/android-push.ts
@@ -42,7 +42,9 @@ export default function AndroidPush() {
       if (endpoint === window.location.pathname) return
       const author = notif?.title
       const message = notif?.body
-      toast.success(`${author}: "${message}"`)
+      // Reuse the endpoint as the toast id so successive messages from the same
+      // conversation replace each other rather than piling up.
+      toast.success(`${author}: "${message}"`, {id: endpoint})
     })
   }, [user?.id, isAndroid])