compose: Automatically change @ mentions to silent mentions in 1:1 DMs.

It is common for users in other chat tools to start off a direct message
with @ mentioning the addressee. However, this results in potentially
unexpected behavior in Zulip, where the message is highlighted and shows
up in @ mentions in addition to DMs. We want to prevent this behaviour by
automatically changing the @ mentions to silent @ mentions for the user.
These changes only apply to 1:1 DMs and not group chats.

Fixes: #17998.

web/src/compose_validate.js

@@ -691,3 +691,22 @@ export function validate(scheduling_message) {
     }
     return validate_stream_message(scheduling_message);
 }
+
+export function convert_mentions_to_silent_in_direct_messages(mention_text, full_name, user_id) {
+    if (compose_state.get_message_type() !== "private") {
+        return mention_text;
+    }
+
+    const recipient_user_id = compose_pm_pill.get_user_ids();
+    if (recipient_user_id.toString() !== user_id.toString()) {
+        return mention_text;
+    }
+
+    const mention_str = people.get_mention_syntax(full_name, user_id, false);
+    const silent_mention_str = people.get_mention_syntax(full_name, user_id, true);
+    mention_text = mention_text.replace(mention_str, silent_mention_str);
+    // also replace other mentions...
+    compose_ui.replace_syntax(mention_str, silent_mention_str);
+
+    return mention_text;
+}

web/src/composebox_typeahead.js

@@ -862,15 +862,20 @@ export function content_typeahead_selected(item, event) {
                 // that functionality yet, and we haven't gotten much
                 // feedback on this being an actual pitfall.
             } else {
-                const mention_text = people.get_mention_syntax(
+                let mention_text = people.get_mention_syntax(
                     item.full_name,
                     item.user_id,
                     is_silent,
                 );
-                beginning += mention_text + " ";
                 if (!is_silent) {
                     compose_validate.warn_if_mentioning_unsubscribed_user(item, $textbox);
+                    mention_text = compose_validate.convert_mentions_to_silent_in_direct_messages(
+                        mention_text,
+                        item.full_name,
+                        item.user_id,
+                    );
                 }
+                beginning += mention_text + " ";
             }
             break;
         }

web/tests/composebox_typeahead.test.js

@@ -476,6 +476,11 @@ test("content_typeahead_selected", ({override}) => {
     fake_this.completing = "mention";
 
     override(compose_validate, "warn_if_mentioning_unsubscribed_user", () => {});
+    override(
+        compose_validate,
+        "convert_mentions_to_silent_in_direct_messages",
+        (mention_text) => mention_text,
+    );
 
     fake_this.query = "@**Mark Tw";
     fake_this.token = "Mark Tw";