feat(buzz): workspace canvas viewer (kind 40100)

Adds a read surface for the Buzz canvas, which was consumed into overlay state
but had no UI. BuzzWorkspaceState gains a canvasUpdates flow; BuzzCanvasScreen
(Route.BuzzCanvas) renders the newest 40100 markdown for a channel and recomposes
when a newer revision lands. Entry point is a Dashboard icon in the relay-group
top bar, shown only on a Buzz-dialect relay once a canvas has arrived.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J8KBSw6smQRyXLiWHeDsZ8

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/navigation/AppNavigation.kt

@@ -102,6 +102,7 @@ import com.vitorpamplona.amethyst.ui.screen.loggedIn.browser.WebAppScreen
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.buzz.AgentAttestationScreen
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.buzz.AgentConsoleScreen
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.buzz.AgentPersonaEditScreen
+import com.vitorpamplona.amethyst.ui.screen.loggedIn.buzz.BuzzCanvasScreen
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.calendars.CalendarCollectionsScreen
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.calendars.CalendarReminderSettingsScreen
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.calendars.CalendarsScreen
@@ -758,6 +759,7 @@ fun BuildNavigation(
                 nav = nav,
             )
         }
+        composableFromEndArgs<Route.BuzzCanvas> { BuzzCanvasScreen(it.channelId, accountViewModel, nav) }
 
         composableFromEndArgs<Route.RelayGroupCreate> {
             RelayGroupCreateScreen(

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/navigation/routes/Routes.kt

@@ -699,6 +699,10 @@ sealed class Route {
         val relayUrl: String,
     ) : Route()
 
+    @Serializable data class BuzzCanvas(
+        val channelId: String,
+    ) : Route()
+
     @Serializable data class RelayGroupCreate(
         val relayUrl: String,
     ) : Route()

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

@@ -0,0 +1,111 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.ui.screen.loggedIn.buzz
+
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.verticalScroll
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Scaffold
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.text.style.TextAlign
+import androidx.compose.ui.unit.dp
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import com.vitorpamplona.amethyst.R
+import com.vitorpamplona.amethyst.commons.model.EmptyTagList
+import com.vitorpamplona.amethyst.commons.model.buzz.BuzzWorkspaceStates
+import com.vitorpamplona.amethyst.ui.components.TranslatableRichTextViewer
+import com.vitorpamplona.amethyst.ui.navigation.navs.INav
+import com.vitorpamplona.amethyst.ui.navigation.topbars.TopBarWithBackButton
+import com.vitorpamplona.amethyst.ui.screen.loggedIn.AccountViewModel
+import com.vitorpamplona.amethyst.ui.stringRes
+
+/**
+ * Renders a Buzz workspace **canvas** (NIP kind 40100): the newest shared markdown
+ * document for a channel. The canvas is overlay state held in [BuzzWorkspaceStates]
+ * (never a timeline row — a workspace has one live canvas, last-write-wins), so this
+ * reads the registry by the channel's `h` id and re-composes off `canvasUpdates` when a
+ * newer revision lands.
+ */
+@Composable
+fun BuzzCanvasScreen(
+    channelId: String,
+    accountViewModel: AccountViewModel,
+    nav: INav,
+) {
+    val state = remember(channelId) { BuzzWorkspaceStates.getOrCreate(channelId) }
+    // Re-read canvasNote whenever a newer canvas is consumed.
+    val version by state.canvasUpdates.collectAsStateWithLifecycle()
+    val canvas = remember(version) { state.canvasNote }
+    val content = canvas?.event?.content
+
+    Scaffold(
+        topBar = { TopBarWithBackButton(stringRes(R.string.buzz_canvas_title), nav) },
+    ) { padding ->
+        if (content.isNullOrBlank() || canvas == null) {
+            Box(
+                modifier = Modifier.padding(padding).fillMaxSize().padding(32.dp),
+                contentAlignment = Alignment.Center,
+            ) {
+                Text(
+                    text = stringRes(R.string.buzz_canvas_empty),
+                    style = MaterialTheme.typography.bodyMedium,
+                    color = MaterialTheme.colorScheme.onSurfaceVariant,
+                    textAlign = TextAlign.Center,
+                )
+            }
+            return@Scaffold
+        }
+
+        val bgColor = remember { mutableStateOf(Color.Transparent) }
+        Box(
+            modifier =
+                Modifier
+                    .padding(padding)
+                    .fillMaxSize()
+                    .verticalScroll(rememberScrollState())
+                    .padding(16.dp),
+        ) {
+            TranslatableRichTextViewer(
+                content = content,
+                canPreview = true,
+                quotesLeft = 1,
+                modifier = Modifier,
+                tags = EmptyTagList,
+                backgroundColor = bgColor,
+                id = canvas.idHex,
+                callbackUri = canvas.toNostrUri(),
+                authorPubKey = canvas.author?.pubkeyHex,
+                accountViewModel = accountViewModel,
+                nav = nav,
+            )
+        }
+    }
+}

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

@@ -37,6 +37,7 @@ import androidx.compose.material3.Surface
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
@@ -50,6 +51,8 @@ import androidx.compose.ui.unit.dp
 import com.vitorpamplona.amethyst.R
 import com.vitorpamplona.amethyst.commons.icons.symbols.Icon
 import com.vitorpamplona.amethyst.commons.icons.symbols.MaterialSymbols
+import com.vitorpamplona.amethyst.commons.model.buzz.BuzzRelayDialect
+import com.vitorpamplona.amethyst.commons.model.buzz.BuzzWorkspaceStates
 import com.vitorpamplona.amethyst.commons.model.nip29RelayGroups.RelayGroupChannel
 import com.vitorpamplona.amethyst.commons.model.nip29RelayGroups.RelayGroupMembership
 import com.vitorpamplona.amethyst.service.relayClient.reqCommand.channel.observeChannel
@@ -164,6 +167,24 @@ fun RelayGroupTopBar(
                 )
             }
 
+            // Buzz workspace canvas (kind 40100): shown only on a Buzz-dialect relay once
+            // a canvas has arrived for this channel. Observing canvasUpdates flips it on
+            // the moment the first canvas is consumed, without swapping the channel object.
+            if (BuzzRelayDialect.isBuzz(channel.groupId.relayUrl)) {
+                val canvasState = remember(channel.groupId.id) { BuzzWorkspaceStates.getOrCreate(channel.groupId.id) }
+                val canvasVersion by canvasState.canvasUpdates.collectAsState()
+                val hasCanvas = remember(canvasVersion) { canvasState.canvasNote != null }
+                if (hasCanvas) {
+                    IconButton(onClick = { nav.nav(Route.BuzzCanvas(channel.groupId.id)) }) {
+                        Icon(
+                            symbol = MaterialSymbols.Dashboard,
+                            contentDescription = stringRes(R.string.buzz_canvas_title),
+                            modifier = Modifier.size(20.dp),
+                        )
+                    }
+                }
+            }
+
             val naddr = channel.toNAddr()
             if (naddr != null) {
                 val context = LocalContext.current

amethyst/src/main/res/values/strings.xml

@@ -3292,6 +3292,8 @@
     <string name="chat_system_updated_channel">%1$s updated the channel profile</string>
     <string name="buzz_message_edited">(edited)</string>
     <string name="buzz_diff_truncated">(diff truncated)</string>
+    <string name="buzz_canvas_title">Canvas</string>
+    <string name="buzz_canvas_empty">No canvas has been shared in this workspace yet.</string>
 
     <string name="chat_delivery_details_title">Message Delivery</string>
     <string name="close">Close</string>

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/model/buzz/BuzzWorkspaceStates.kt

@@ -56,6 +56,11 @@ class BuzzWorkspaceState {
     var canvasNote: Note? = null
         private set
 
+    private val canvasVersion = MutableStateFlow(0)
+
+    /** Bumps when [canvasNote] is replaced by a newer revision, so a canvas view re-reads it. */
+    val canvasUpdates: StateFlow<Int> = canvasVersion
+
     /** Records a 40003 edit; keeps only the newest per target (last-write-wins by created_at). */
     fun addEdit(
         targetId: HexKey,
@@ -78,6 +83,7 @@ class BuzzWorkspaceState {
         lock.withLock {
             if ((note.createdAt() ?: 0L) > (canvasNote?.createdAt() ?: 0L)) {
                 canvasNote = note
+                canvasVersion.value = canvasVersion.value + 1
             }
         }