Add hotkey for narrowing to next unread topic.

Fixes: #4199.

frontend_tests/node_tests/hotkey.js

@@ -245,6 +245,16 @@ function stubbing(func_name_to_stub, test_function) {
     assert_mapping(':', 'emoji_picker.toggle_emoji_popover', true);
     assert_mapping('G', 'navigate.to_end');
     assert_mapping('M', 'muting_ui.toggle_mute');
+
+    // Test keys that work when a message is selected and
+    // also when the message list is empty.
+    assert_mapping('n', 'narrow.narrow_to_next_topic');
+
+    global.current_msg_list.empty = return_true;
+    assert_mapping('n', 'narrow.narrow_to_next_topic');
+
+    global.current_msg_list.empty = return_false;
+
 }());
 
 (function test_motion_keys() {

static/js/hotkey.js

@@ -115,7 +115,7 @@ var keypress_mappings = {
     105: {name: 'message_actions', message_view_only: true}, // 'i'
     106: {name: 'vim_down', message_view_only: true}, // 'j'
     107: {name: 'vim_up', message_view_only: true}, // 'k'
-    110: {name: 'new_stream', message_view_only: false}, // 'n'
+    110: {name: 'n_key', message_view_only: false}, // 'n'
     113: {name: 'query_users', message_view_only: false}, // 'q'
     114: {name: 'reply_message', message_view_only: true}, // 'r'
     115: {name: 'narrow_by_recipient', message_view_only: true}, // 's'
@@ -591,9 +591,11 @@ exports.process_hotkey = function (e, hotkey) {
                 subs.view_stream();
             }
             return true;
-        case 'new_stream':
+        case 'n_key':
             if (exports.is_subs()) {
                 subs.new_stream_clicked();
+            } else {
+                narrow.narrow_to_next_topic();
             }
             return true;
         case 'open_drafts':

templates/zerver/help/keyboard-shortcuts.md

@@ -69,6 +69,8 @@ Zulip keyboard shortcuts are divided into four categories:
   messages with the topic of the selected message (outlined in blue).
 * **Narrow to all private messages**: `P` - This shortcut narrows the
   view to show all of the user's private messages.
+* **Narrow to next unread topic**: `n` - This shortcut narrows the
+  view to the next unread topic in the stream sidebar.
 * **Cycle between stream narrows**: `A` and `D` - This shortcut allows the
   user to cycle through the narrows showing the messages of a stream
   according to Stream order in the left sidebar. `A` allows the user

templates/zerver/keyboard_shortcuts.html

@@ -106,6 +106,10 @@
           <td class="hotkey">P</td>
           <td class="definition">{% trans %}Narrow to all private messages{% endtrans %}</td>
         </tr>
+        <tr>
+          <td class="hotkey">n</td>
+          <td class="definition">{% trans %}Narrow to next unread topic{% endtrans %}</td>
+        </tr>
         <tr>
           <td class="hotkey">A or D</td>
           <td class="definition">{% trans %}Cycle between stream narrows{% endtrans %}</td>