NEXUS-4082: Keep the new-message boundary stable during a visit

docs/buzz-help.md

@@ -160,6 +160,13 @@ that agent in Buzz; they do not notify the external app through this connection.
 - **Inbox** collects DMs, ordinary responses in Solo chats, mentions of you, and replies in threads you started,
   replied in, or follow. Solo chats appear under All, not DMs; their nested threads stay separate. Agent-only
   coordination stays out.
+- **New messages** marks the first known unread incoming channel message when
+  you enter. Once established, the divider stays in place for that visit as you
+  read, reveal another message, or receive new messages. It is absent for a visit
+  that starts with incomplete or unverified history, even if older rows load
+  later. It may also be absent when unread evidence is incomplete, or when read
+  intent or the read evidence needed to locate the boundary changes before it is
+  established.
 - **Communities** have their own channels, people, and agents. Switching
   communities changes the available context. A channel reopens where you
   stopped reading, or at the latest message once you had caught up. You and

src/bundled/channels/ChannelsPage.solo.test.tsx

@@ -222,6 +222,11 @@ function fixture({
     coverage: "observed" as const,
     freshness: "observed" as const,
   };
+  const readSync = {
+    capability,
+    status: "reconciled" as const,
+    completeness: "snapshot" as const,
+  };
   const session = {
     ...owner.session,
     profiles: {
@@ -293,11 +298,7 @@ function fixture({
         ? owner.session.unread
         : {
             ...owner.session.unread,
-            sync: () => ({
-              capability,
-              status: "reconciled" as const,
-              completeness: "snapshot" as const,
-            }),
+            sync: () => readSync,
             snapshot: (
               target: Parameters<typeof owner.session.unread.snapshot>[0],
             ) => {

src/bundled/channels/ConversationTab.test.tsx

@@ -84,6 +84,10 @@ vi.mock("../../features/messages/MessageComposer", () => ({
     </div>
   ),
 }));
+const unreadEvidence = {
+  revision: () => 0,
+  sync: () => ({ capability: "unsupported" }),
+};
 const owners: ReturnType<typeof createOutbox>[] = [];
 afterEach(() => {
   cleanup();
@@ -138,8 +142,13 @@ it.each(["thread", "conversation"] as const)(
     // Only management consumes this session; independent child views above are
     // stubbed. These writable tabs have static eligibility/profile evidence.
     const session = {
+      unread: unreadEvidence,
       outbox: owner.outbox,
-      channels: { list: () => channels, subscribeList: () => () => {} },
+      channels: {
+        window: () => ({ rows: [] }),
+        list: () => channels,
+        subscribeList: () => () => {},
+      },
       profiles: { subscribe: () => () => {} },
       messages: { canRetry: () => true },
     } as unknown as RelaySession;
@@ -201,9 +210,13 @@ it.each(["thread", "conversation"] as const)(
     const leaveChannel = vi.fn();
     const session = {
       viewer: "viewer",
-      channels: { list: () => snapshot, subscribeList: () => () => {} },
+      channels: {
+        window: () => ({ rows: [] }),
+        list: () => snapshot,
+        subscribeList: () => () => {},
+      },
       profiles: {},
-      unread: { enterChannel, leaveChannel },
+      unread: { ...unreadEvidence, enterChannel, leaveChannel },
     } as unknown as RelaySession;
     const props = {
       tab: {
@@ -253,9 +266,13 @@ it.each(["thread", "conversation"] as const)(
     const enterChannel = vi.fn(async () => {});
     const session = {
       viewer: "viewer",
-      channels: { list: () => snapshot, subscribeList: () => () => {} },
+      channels: {
+        window: () => ({ rows: [] }),
+        list: () => snapshot,
+        subscribeList: () => () => {},
+      },
       profiles: {},
-      unread: { enterChannel, leaveChannel: vi.fn() },
+      unread: { ...unreadEvidence, enterChannel, leaveChannel: vi.fn() },
     } as unknown as RelaySession;
     render(
       <ConversationTab
@@ -294,7 +311,12 @@ it.each([false, true])(
     const channel = { id: "beta", name: "Beta" };
     const snapshot = { status: "ready", channels: [channel] };
     const session = {
-      channels: { list: () => snapshot, subscribeList: () => () => {} },
+      unread: unreadEvidence,
+      channels: {
+        window: () => ({ rows: [] }),
+        list: () => snapshot,
+        subscribeList: () => () => {},
+      },
     } as unknown as RelaySession;
     const tab = {
       id: "thread",
@@ -340,7 +362,12 @@ it("a tab mounted later in its visit treats the visit's requests as restored", (
   const channel = { id: "beta", name: "Beta" };
   const snapshot = { status: "ready", channels: [channel] };
   const session = {
-    channels: { list: () => snapshot, subscribeList: () => () => {} },
+    unread: unreadEvidence,
+    channels: {
+      window: () => ({ rows: [] }),
+      list: () => snapshot,
+      subscribeList: () => () => {},
+    },
   } as unknown as RelaySession;
   const tab: Tab = {
     id: "thread",
@@ -390,7 +417,12 @@ it.each([
     const channel = { id: "beta", name: "Beta" };
     const snapshot = { status: "ready", channels: [channel] };
     const session = {
-      channels: { list: () => snapshot, subscribeList: () => () => {} },
+      unread: unreadEvidence,
+      channels: {
+        window: () => ({ rows: [] }),
+        list: () => snapshot,
+        subscribeList: () => () => {},
+      },
     } as unknown as RelaySession;
     const tab: Tab =
       kind === "thread"
@@ -434,7 +466,12 @@ it.each(["pointerDown", "keyDown"] as const)(
     const channel = { id: "beta", name: "Beta" };
     const snapshot = { status: "ready", channels: [channel] };
     const session = {
-      channels: { list: () => snapshot, subscribeList: () => () => {} },
+      unread: unreadEvidence,
+      channels: {
+        window: () => ({ rows: [] }),
+        list: () => snapshot,
+        subscribeList: () => () => {},
+      },
     } as unknown as RelaySession;
     const tab: Tab = {
       id: "thread",
@@ -482,6 +519,7 @@ function openingAt(rows: { id: string; threadRootId?: string }[]) {
   };
   const listeners = new Set<() => void>();
   const session = {
+    unread: unreadEvidence,
     channels: {
       list: () => snapshot,
       subscribeList: () => () => {},
@@ -641,7 +679,12 @@ it("lets the tab supply Solo chat identity without repeating a visible heading",
   const idle = () => () => {};
   const profiles = new Map();
   const session = {
-    channels: { list: () => snapshot, subscribeList: idle },
+    unread: unreadEvidence,
+    channels: {
+      window: () => ({ rows: [] }),
+      list: () => snapshot,
+      subscribeList: idle,
+    },
     profiles: { snapshot: () => profiles, subscribe: idle },
     agentChoices: { snapshot: () => undefined, subscribe: idle },
   } as unknown as RelaySession;
@@ -675,7 +718,12 @@ it("tells the composer when its tab is shown, apart from whether showing visits"
   const idle = () => () => {};
   const profiles = new Map();
   const session = {
-    channels: { list: () => snapshot, subscribeList: idle },
+    unread: unreadEvidence,
+    channels: {
+      window: () => ({ rows: [] }),
+      list: () => snapshot,
+      subscribeList: idle,
+    },
     profiles: { snapshot: () => profiles, subscribe: idle },
     agentChoices: { snapshot: () => undefined, subscribe: idle },
   } as unknown as RelaySession;

src/bundled/channels/useConversationTabs.test.tsx

@@ -1870,6 +1870,7 @@ it("a hidden retained thread tab's rows re-render for new panels, not channel-li
     messages: { retry: () => {} },
     media: () => undefined,
     unread: {
+      revision: () => 0,
       sync: () => sync,
       snapshot: () => undefined,
       subscribe: () => () => {},

src/features/conversation/EmbeddedThread.test.tsx

@@ -143,6 +143,7 @@ function fixture(
     ],
   };
   const unread = {
+    revision: () => 0,
     sync: () => ({ capability: "unsupported" }),
     snapshot: () => undefined,
     subscribe: () => () => {},
@@ -166,7 +167,11 @@ function fixture(
       loadMore: async () => {},
       dispose() {},
     }),
-    channels: { subscribeList: () => () => {}, list: () => channels },
+    channels: {
+      subscribeList: () => () => {},
+      list: () => channels,
+      window: () => ({ rows: [] }),
+    },
     profiles: {
       ensure: async () => {},
       snapshot: () => profiles,