combined-feed-ui: Add placeholder text to empty combined feed.

Also, added live update behaviour to the combined feed narrow.

Fixes #31602.

web/src/message_list.ts

@@ -579,7 +579,6 @@ export class MessageList {
         this.view.clear_rendering_state(false);
         this.view.update_render_window(this.selected_idx(), false);
 
-        if (!this.is_combined_feed_view) {
         if (
             this.visibly_empty() &&
             this.data.fetch_status.has_found_oldest() &&
@@ -592,7 +591,6 @@ export class MessageList {
         } else {
             narrow_banner.hide_empty_narrow_message();
         }
-        }
         this.rerender_view();
     }
 

web/src/narrow_banner.ts

@@ -128,10 +128,28 @@ export function pick_empty_narrow_banner(): NarrowBannerData {
     const default_banner_for_multiple_filters = $t({defaultMessage: "No search results."});
 
     const current_filter = narrow_state.filter();
-
+    if (current_filter === undefined) {
-    if (current_filter === undefined || current_filter.is_in_home()) {
+        // We're in either the inbox or recent conversations view.
         return default_banner;
     }
+    if (current_filter.is_in_home()) {
+        // We're in the combined feed view.
+        return {
+            title: $t({defaultMessage: "There are no messages in your combined feed."}),
+            html: page_params.is_spectator
+                ? ""
+                : $t_html(
+                      {
+                          defaultMessage:
+                              "Would you like to <z-link>view messages in all public channels</z-link>?",
+                      },
+                      {
+                          "z-link": (content_html) =>
+                              `<a href="#narrow/channels/public">${content_html.join("")}</a>`,
+                      },
+                  ),
+        };
+    }
 
     const first_term = current_filter.terms()[0]!;
     const current_terms_types = current_filter.sorted_term_types();

web/tests/message_view.test.cjs

@@ -248,6 +248,18 @@ run_test("show_empty_narrow_message", ({mock_template, override}) => {
         ),
     );
 
+    // for empty combined feed
+    const current_filter = new Filter([{operator: "in", operand: "home"}]);
+    message_lists.set_current({data: {filter: current_filter}});
+    narrow_banner.show_empty_narrow_message();
+    assert.equal(
+        $(".empty_feed_notice_main").html(),
+        empty_narrow_html(
+            "translated: There are no messages in your combined feed.",
+            'translated HTML: Would you like to <a href="#narrow/channels/public">view messages in all public channels</a>?',
+        ),
+    );
+
     // for non-existent or private stream
     set_filter([["stream", "999"]]);
     narrow_banner.show_empty_narrow_message();