feat: warm each group's recent messages on a relay's channel list

Opening a group from the relay's channel list used to start its chat from a cold
load. Mount the existing RelayGroupWarmupSubscription on every visible card
(content-only — the directory subscription already streams metadata), so a tap
lands on already-cached messages.

Add a contentLimit to the warmup (default 50, unchanged for discovery's "50+"
signal); the channel list passes ~10 — a first screen's worth. Bounded to
visible rows by the LazyColumn and released as they scroll off.

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/chats/publicChannels/relayGroup/RelayGroupChannelListScreen.kt

@@ -60,6 +60,7 @@ import com.vitorpamplona.amethyst.ui.navigation.routes.Route
 import com.vitorpamplona.amethyst.ui.navigation.routes.routeFor
 import com.vitorpamplona.amethyst.ui.navigation.topbars.TopBarExtensibleWithBackButton
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.AccountViewModel
+import com.vitorpamplona.amethyst.ui.screen.loggedIn.chats.publicChannels.relayGroup.datasource.RelayGroupWarmupSubscription
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.chats.publicChannels.relayGroup.datasource.RelayGroupsOnRelaySubscription
 import com.vitorpamplona.amethyst.ui.stringRes
 import com.vitorpamplona.quartz.nip01Core.relay.filters.Filter
@@ -67,11 +68,15 @@ import com.vitorpamplona.quartz.nip01Core.relay.normalizer.RelayUrlNormalizer
 import com.vitorpamplona.quartz.nip01Core.relay.normalizer.displayUrl
 import com.vitorpamplona.quartz.nip29RelayGroups.metadata.GroupMetadataEvent
 
+/** A first screen's worth of recent messages to prefetch per visible group card, ahead of a tap. */
+private const val CHANNEL_LIST_WARMUP_LIMIT = 10
+
 /**
  * Lists every channel a relay hosts (its kind 39000-39003 directory), so the user
  * can browse and open channels on that relay. The relay's directory is streamed by
  * [RelayGroupsOnRelaySubscription] and consumed into per-group channels; this
- * screen reads them back for the relay and renders them.
+ * screen reads them back for the relay and renders them. Each visible card also warms
+ * its group's recent messages so opening a chat lands on cached content.
  */
 @Composable
 fun RelayGroupChannelListScreen(
@@ -156,6 +161,18 @@ private fun RelayGroupChannelRow(
     val joined = channel.membershipOf(myPubkey).isMember()
     val memberCount = channel.memberCount()
 
+    // Anticipate a tap: while this row is on-screen, prefetch a first screen's worth of recent
+    // messages for its group (content only — the directory subscription already streams metadata),
+    // so opening the chat lands on cached content instead of a blank load. Bounded to visible rows
+    // by the LazyColumn, and released as they scroll off.
+    RelayGroupWarmupSubscription(
+        channel,
+        accountViewModel.dataSources().relayGroupWarmup,
+        accountViewModel,
+        contentOnly = true,
+        contentLimit = CHANNEL_LIST_WARMUP_LIMIT,
+    )
+
     Row(
         modifier =
             Modifier

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/publicChannels/relayGroup/datasource/RelayGroupWarmupFilterAssembler.kt

@@ -40,6 +40,8 @@ class RelayGroupWarmupQueryState(
     val channel: RelayGroupChannel,
     /** When true, prefetch only recent content — the caller's screen already streams metadata. */
     val contentOnly: Boolean = false,
+    /** How many recent content events to prefetch ahead of a tap. */
+    val contentLimit: Int = RELAY_GROUP_WARMUP_LIMIT,
 )
 
 /** Newest content kinds we prefetch so opening the card lands on populated screens. */
@@ -47,11 +49,12 @@ private val RELAY_GROUP_WARMUP_CONTENT_KINDS =
     listOf(ChatEvent.KIND, PollEvent.KIND, ThreadEvent.KIND, CommentEvent.KIND)
 
 /**
- * How many recent events to pull ahead of a tap — enough to fill the first screen AND drive the
- * discovery card's "50+ messages" activity signal (a chat that returns the full page reads as
- * "50+"; fewer shows the exact loaded count).
+ * Default number of recent events to pull ahead of a tap — enough to fill the first screen AND drive
+ * the discovery card's "50+ messages" activity signal (a chat that returns the full page reads as
+ * "50+"; fewer shows the exact loaded count). Callers that only need a first-screen preview (e.g. a
+ * relay's channel list) pass a smaller [RelayGroupWarmupQueryState.contentLimit].
  */
-private const val RELAY_GROUP_WARMUP_LIMIT = 50
+const val RELAY_GROUP_WARMUP_LIMIT = 50
 
 /**
  * Warms a NIP-29 group referenced inline (a group-link card) without opening it:
@@ -92,7 +95,7 @@ class RelayGroupWarmupSubAssembler(
                     Filter(
                         kinds = RELAY_GROUP_WARMUP_CONTENT_KINDS,
                         tags = mapOf(GroupIdTag.TAG_NAME to listOf(groupId.id)),
-                        limit = RELAY_GROUP_WARMUP_LIMIT,
+                        limit = key.contentLimit,
                         since = since?.get(groupId.relayUrl)?.time,
                     ),
             )

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/publicChannels/relayGroup/datasource/RelayGroupWarmupSubscription.kt

@@ -37,10 +37,11 @@ fun RelayGroupWarmupSubscription(
     dataSource: RelayGroupWarmupFilterAssembler,
     accountViewModel: AccountViewModel,
     contentOnly: Boolean = false,
+    contentLimit: Int = RELAY_GROUP_WARMUP_LIMIT,
 ) {
     val state =
-        remember(channel.groupId, contentOnly) {
-            RelayGroupWarmupQueryState(channel, contentOnly)
+        remember(channel.groupId, contentOnly, contentLimit) {
+            RelayGroupWarmupQueryState(channel, contentOnly, contentLimit)
         }
 
     LifecycleAwareKeyDataSourceSubscription(state, dataSource)