fix: make start messages stick to the top of the chat

We already set sort_timestamp to 0 for "Messages are end-to-end encrypted."
since 8f1bf963b4.
Do this for "Others will only see this group after you sent a first message."
and "Messages in this chat use classic email and are not encrypted." as well
so no messages can be added on top.

src/chat.rs

@@ -497,6 +497,27 @@ impl ChatId {
         Ok(())
     }
 
+    /// Adds info message to the beginning of the chat.
+    ///
+    /// Used for messages such as
+    /// "Others will only see this group after you sent a first message."
+    pub(crate) async fn add_start_info_message(self, context: &Context, text: &str) -> Result<()> {
+        let sort_timestamp = 0;
+        add_info_msg_with_cmd(
+            context,
+            self,
+            text,
+            SystemMessage::Unknown,
+            Some(sort_timestamp),
+            time(),
+            None,
+            None,
+            None,
+        )
+        .await?;
+        Ok(())
+    }
+
     /// Archives or unarchives a chat.
     pub async fn set_visibility(self, context: &Context, visibility: ChatVisibility) -> Result<()> {
         self.set_visibility_ex(context, Sync, visibility).await
@@ -3611,7 +3632,7 @@ pub(crate) async fn create_group_ex(
             // Add "Messages in this chat use classic email and are not encrypted." message.
             stock_str::chat_unencrypted_explanation(context)
         };
-        add_info_msg(context, chat_id, &text).await?;
+        chat_id.add_start_info_message(context, &text).await?;
     }
     if let (true, true) = (sync.into(), !grpid.is_empty()) {
         let id = SyncId::Grpid(grpid);

src/chat/chat_tests.rs

@@ -7,7 +7,7 @@ use crate::constants::{DC_GCL_ARCHIVED_ONLY, DC_GCL_NO_SPECIALS};
 use crate::ephemeral::Timer;
 use crate::headerdef::HeaderDef;
 use crate::imex::{ImexMode, has_backup, imex};
-use crate::message::{MessengerMessage, delete_msgs};
+use crate::message::{Message, MessengerMessage, delete_msgs};
 use crate::mimeparser::{self, MimeMessage};
 use crate::receive_imf::receive_imf;
 use crate::securejoin::{get_securejoin_qr, join_securejoin};
@@ -6156,3 +6156,67 @@ async fn test_late_message_above_seen() -> Result<()> {
 
     Ok(())
 }
+
+/// Tests that start message for unpromoted groups sticks to the top of the chat.
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_unpromoted_group_start_message() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let alice = &tcm.alice().await;
+    let bob = &tcm.bob().await;
+
+    // Start messages are disabled for test context by default,
+    // but this test is specifically about start messages.
+    alice.set_config(Config::SkipStartMessages, None).await?;
+
+    // Shift the clock forward, so we can rewind it back later.
+    SystemTime::shift(Duration::from_secs(3600));
+
+    // Alice creates unpromoted group with Bob.
+    let chat_id = create_group(alice, "Group").await?;
+    let bob_id = alice.add_or_lookup_contact_id(bob).await;
+    add_contact_to_chat(alice, chat_id, bob_id).await?;
+
+    let [
+        ChatItem::Message {
+            msg_id: e2ee_msg_id,
+        },
+        ChatItem::Message {
+            msg_id: info_msg_id,
+        },
+    ] = get_chat_msgs(alice, chat_id).await?[..]
+    else {
+        panic!("Expected two messages in the chat");
+    };
+    let msg = Message::load_from_db(alice, e2ee_msg_id).await?;
+    assert_eq!(msg.text, "Messages are end-to-end encrypted.");
+    let msg = Message::load_from_db(alice, info_msg_id).await?;
+    assert_eq!(
+        msg.text,
+        "Others will only see this group after you sent a first message."
+    );
+
+    // Alice rewinds the clock.
+    SystemTime::shift_back(Duration::from_secs(3600));
+
+    let text_msg_id = send_text_msg(alice, chat_id, "Hello".to_string()).await?;
+
+    let [
+        ChatItem::Message {
+            msg_id: e2ee_msg_id2,
+        },
+        ChatItem::Message {
+            msg_id: info_msg_id2,
+        },
+        ChatItem::Message {
+            msg_id: text_msg_id2,
+        },
+    ] = get_chat_msgs(alice, chat_id).await?[..]
+    else {
+        panic!("Expected three messages in the chat");
+    };
+    assert_eq!(e2ee_msg_id2, e2ee_msg_id);
+    assert_eq!(info_msg_id2, info_msg_id);
+    assert_eq!(text_msg_id2, text_msg_id);
+
+    Ok(())
+}