test(chat): cover the unread marker boundary latch

Extracts the latch condition from buildChatItems into the pure
findFirstUnreadMessageId (no behavior change) and covers it: the marker
position is only derived when a message at or below lastReadMessage is
visible, windows floating entirely above the boundary and temporary
messages with negative ids yield no marker, and a deleted or expired
boundary message is handled by any older visible message.

Assisted-by: Claude Code:claude-fable-5
Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

Author: AndyScherzinger

app/src/main/java/com/nextcloud/talk/chat/viewmodels/ChatViewModel.kt

@@ -1137,22 +1137,10 @@ class ChatViewModel @AssistedInject constructor(
 
         return buildList {
             if (firstUnreadMessageId == null && lastReadMessage > 0) {
-                // Latch the marker position only when the visible window provably reaches back to
-                // the unread boundary, i.e. a message at or below lastReadMessage is visible.
-                // Without that proof the oldest visible message may still be far above the true
-                // first unread message (e.g. after a capped fetch of only the newest messages) and
-                // the marker would be latched in the middle of the unread messages. Temporary
-                // messages carry negative ids and don't count as proof.
-                val unreadBoundaryIsVisible = uiMessages.any { it.id in 1..lastReadMessage }
-                if (unreadBoundaryIsVisible) {
-                    firstUnreadMessageId =
-                        uiMessages.firstOrNull {
-                            it.id > lastReadMessage
-                        }?.id
-                    Log.d(TAG, "reversedMessages.size = ${uiMessages.size}")
-                    Log.d(TAG, "firstUnreadMessageId = $firstUnreadMessageId")
-                    Log.d(TAG, "conversation.lastReadMessage = $lastReadMessage")
-                }
+                firstUnreadMessageId = findFirstUnreadMessageId(uiMessages, lastReadMessage)
+                Log.d(TAG, "reversedMessages.size = ${uiMessages.size}")
+                Log.d(TAG, "firstUnreadMessageId = $firstUnreadMessageId")
+                Log.d(TAG, "conversation.lastReadMessage = $lastReadMessage")
             }
 
             for (uiMessage in uiMessages) {
@@ -2399,6 +2387,25 @@ class ChatViewModel @AssistedInject constructor(
 
     companion object {
         private val TAG = ChatViewModel::class.simpleName
+
+        /**
+         * Returns the id of the first unread message, or null when it cannot be determined (yet).
+         *
+         * The position is only trustworthy when the visible window provably reaches back to the
+         * unread boundary, i.e. a message at or below [lastReadMessage] is visible. Without that
+         * proof the oldest visible message may still be far above the true first unread message
+         * (e.g. after a capped fetch of only the newest messages) and a marker latched onto it
+         * would sit in the middle of the unread messages. Temporary messages carry negative ids
+         * and don't count as proof.
+         */
+        internal fun findFirstUnreadMessageId(uiMessages: List<ChatMessageUi>, lastReadMessage: Int): Int? {
+            val unreadBoundaryIsVisible = uiMessages.any { it.id in 1..lastReadMessage }
+            if (!unreadBoundaryIsVisible) {
+                return null
+            }
+            return uiMessages.firstOrNull { it.id > lastReadMessage }?.id
+        }
+
         const val JOIN_ROOM_RETRY_COUNT: Long = 3
         const val HTTP_CODE_OK: Int = 200
         private const val CONVERSATION_AND_USER_FLOW_SHARING_TIMEOUT_MS = 5_000L

app/src/test/java/com/nextcloud/talk/chat/viewmodels/ChatViewModelTest.kt

@@ -0,0 +1,80 @@
+/*
+ * Nextcloud Talk - Android Client
+ *
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+package com.nextcloud.talk.chat.viewmodels
+
+import com.nextcloud.talk.chat.ui.model.ChatMessageUi
+import com.nextcloud.talk.chat.ui.model.MessageStatusIcon
+import com.nextcloud.talk.chat.ui.model.MessageTypeContent
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+import java.time.LocalDate
+
+/**
+ * Covers the unread marker latch: the marker position must only be derived from the visible
+ * window when the window provably reaches back to the unread boundary — otherwise a window of
+ * only-unread messages (e.g. after a capped fetch of the newest messages) would place the marker
+ * in the middle of the unread messages.
+ */
+class ChatViewModelTest {
+
+    @Test
+    fun `marker is placed at the first message above the boundary when the boundary is visible`() {
+        val messages = listOf(uiMessage(39), uiMessage(40), uiMessage(41), uiMessage(42))
+
+        assertEquals(41, ChatViewModel.findFirstUnreadMessageId(messages, lastReadMessage = 40))
+    }
+
+    @Test
+    fun `marker is placed correctly when the boundary message itself is missing`() {
+        // the last read message may have been deleted or expired — an older read message is
+        // equally valid proof that the window reaches the boundary
+        val messages = listOf(uiMessage(38), uiMessage(41), uiMessage(42))
+
+        assertEquals(41, ChatViewModel.findFirstUnreadMessageId(messages, lastReadMessage = 40))
+    }
+
+    @Test
+    fun `no marker is placed when the window floats entirely above the boundary`() {
+        val messages = listOf(uiMessage(141), uiMessage(142), uiMessage(143))
+
+        assertNull(ChatViewModel.findFirstUnreadMessageId(messages, lastReadMessage = 40))
+    }
+
+    @Test
+    fun `temporary messages with negative ids are no proof of the boundary`() {
+        val messages = listOf(uiMessage(-5), uiMessage(141), uiMessage(142))
+
+        assertNull(ChatViewModel.findFirstUnreadMessageId(messages, lastReadMessage = 40))
+    }
+
+    @Test
+    fun `no marker is placed when everything is read`() {
+        val messages = listOf(uiMessage(38), uiMessage(39), uiMessage(40))
+
+        assertNull(ChatViewModel.findFirstUnreadMessageId(messages, lastReadMessage = 40))
+    }
+
+    private fun uiMessage(id: Int): ChatMessageUi =
+        ChatMessageUi(
+            id = id,
+            message = "message $id",
+            renderMarkdown = false,
+            actorDisplayName = "Other User",
+            isThread = false,
+            threadTitle = "",
+            threadReplies = 0,
+            incoming = true,
+            isDeleted = false,
+            avatarUrl = null,
+            statusIcon = MessageStatusIcon.SENT,
+            timestamp = id.toLong(),
+            date = LocalDate.of(2026, 8, 12),
+            content = MessageTypeContent.RegularText
+        )
+}