fix: show a just-joined NIP-29 group on the Messages tab immediately

The Messages tab (INLINE mode) mapped each joined group to its newest cached
kind-9 message and dropped it when none existed. Since the joined-groups
subscription only fetches roster kinds (39000/1/2), not chat, a group you just
joined stayed invisible until you opened it (loading messages) or posted — unlike
Marmot groups, which already fall back to a placeholder row.

Mirror the Marmot pattern for relay groups:
 - RelayGroupChannel.placeholderNote(): a cached synthetic note that adds the
   channel as a gatherer, so the existing Messages row renderer resolves it back
   to the group (RelayGroupRoomCompose already handles a null-event note).
 - ChatroomListKnownFeedFilter.feed(): fall back to placeholderNote() when the
   group has no loaded message.
 - AccountFeedContentStates: rebuild dmKnown when relayGroupList (kind 10009)
   changes — join/leave doesn't flow through newEventBundles, so without this the
   placeholder wouldn't appear until a later event.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B5MLY4hq5LXJ2D5WeLRyXj

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/AccountFeedContentStates.kt

@@ -167,6 +167,17 @@ class AccountFeedContentStates(
             }
         }
 
+        // Same for the NIP-29 joined-group list (kind 10009): joining/leaving changes the list but
+        // doesn't flow through newEventBundles, so force a rebuild — otherwise a just-joined group
+        // (whose messages haven't loaded yet) wouldn't appear on the Messages tab until a later event.
+        scope.launch(Dispatchers.IO) {
+            account.relayGroupList.liveRelayGroupList
+                .drop(1)
+                .collect {
+                    dmKnown.invalidateData()
+                }
+        }
+
         // Pinning/unpinning a room only changes sort order, not membership, so no
         // chat event flows through LocalCache. Force a rebuild to re-sort. This
         // also fires when pins arrive via the synced AppSpecificData event.

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/rooms/dal/ChatroomListKnownFeedFilter.kt

@@ -102,12 +102,14 @@ class ChatroomListKnownFeedFilter(
             if (account.settings.relayGroupViewMode.value == RelayGroupViewMode.INLINE) {
                 account.relayGroupList.liveRelayGroupList.value.mapNotNull { groupTag ->
                     val relay = RelayUrlNormalizer.normalizeOrNull(groupTag.relayUrl) ?: return@mapNotNull null
-                    LocalCache
-                        .getOrCreateRelayGroupChannel(GroupId(groupTag.groupId, relay))
-                        .notes
+                    val channel = LocalCache.getOrCreateRelayGroupChannel(GroupId(groupTag.groupId, relay))
+                    // Newest loaded message, or a placeholder row so a just-joined group shows up on
+                    // Messages before its first kind-9 arrives (mirrors the Marmot-group path above).
+                    channel.notes
                         .filter { _, it -> account.isAcceptable(it) && it.event != null }
                         .sortedByDefaultFeedOrder()
                         .firstOrNull()
+                        ?: channel.placeholderNote()
                 }
             } else {
                 emptyList()

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/model/nip29RelayGroups/RelayGroupChannel.kt

@@ -23,6 +23,8 @@ package com.vitorpamplona.amethyst.commons.model.nip29RelayGroups
 import androidx.compose.runtime.Stable
 import com.vitorpamplona.amethyst.commons.model.Channel
 import com.vitorpamplona.amethyst.commons.model.Note
+import com.vitorpamplona.amethyst.commons.util.KmpLock
+import com.vitorpamplona.amethyst.commons.util.withLock
 import com.vitorpamplona.quartz.nip01Core.core.HexKey
 import com.vitorpamplona.quartz.nip19Bech32.entities.NAddress
 import com.vitorpamplona.quartz.nip29RelayGroups.GroupId
@@ -203,4 +205,24 @@ class RelayGroupChannel(
         groupId.id.contains(prefix, true) ||
             event?.name()?.contains(prefix, true) == true ||
             event?.about()?.contains(prefix, true) == true
+
+    // Synthetic note representing this group in list views (the Messages tab) before any message
+    // has loaded — so a group the user just joined shows up immediately instead of waiting for its
+    // first cached kind-9. Mirrors MarmotGroupChatroom.placeholderNote(): adds this channel as a
+    // gatherer so the list row resolves back to it, and is cached with a stable id so equality-based
+    // feed diffing treats it as the same row across refreshes.
+    private val placeholderLock = KmpLock()
+    private var cachedPlaceholder: Note? = null
+
+    fun placeholderNote(): Note =
+        placeholderLock.withLock {
+            cachedPlaceholder ?: Note(placeholderIdHex(groupId)).apply {
+                addGatherer(this@RelayGroupChannel)
+                cachedPlaceholder = this
+            }
+        }
+
+    companion object {
+        fun placeholderIdHex(groupId: GroupId): HexKey = "relaygroup-empty-${groupId.toKey()}"
+    }
 }