narrow_banner: Differentiate between unknown channel and new topic.

web/src/narrow_banner.ts

@@ -192,7 +192,21 @@ export function pick_empty_narrow_banner(): NarrowBannerData {
             return SPECTATOR_STREAM_NARROW_BANNER;
         }
 
-        // A stream > topic that doesn't exist yet.
+        if (streams.length === 1) {
+            const stream_sub = stream_data.get_sub_by_id_string(
+                util.the(current_filter.operands("channel")),
+            );
+            if (!stream_sub) {
+                return {
+                    title: $t({
+                        defaultMessage:
+                            "This channel doesn't exist, or you are not allowed to view it.",
+                    }),
+                };
+            }
+        }
+
+        // A valid stream, but a topic that doesn't exist yet.
         if (num_terms === 2 && streams.length === 1 && topics.length === 1) {
             return default_banner;
         }

web/tests/message_view.test.js

@@ -258,6 +258,19 @@ run_test("show_empty_narrow_message", ({mock_template, override}) => {
         ),
     );
 
+    set_filter([
+        ["stream", "999"],
+        ["topic", "foo"],
+        ["near", "99"],
+    ]);
+    narrow_banner.show_empty_narrow_message();
+    assert.equal(
+        $(".empty_feed_notice_main").html(),
+        empty_narrow_html(
+            "translated: This channel doesn't exist, or you are not allowed to view it.",
+        ),
+    );
+
     // for non-subbed public stream
     const rome_id = 99;
     stream_data.add_sub({name: "ROME", stream_id: rome_id});