MDL-64058 core_message: fix logic driving conversation section expansion

Unread conversations should be given priority over read. If one or more
sections have unread conversations, prioritise favourites over group and
group over individual. The same applies if all conversations are read.

message/lib.php

@@ -869,6 +869,22 @@ function core_message_standard_after_main_region_html() {
     // Get the unread counts for the current user.
     $unreadcounts = \core_message\api::get_unread_conversation_counts($USER->id);
 
+    // Determine which section will be expanded.
+    // Default behaviour - if no unread counts exist.
+    $favouritesexpanded = !empty($favouriteconversationcount);
+    $groupmessagesexpanded = empty($favouriteconversationcount) && !empty($groupconversationcount);
+    $messagesexpanded = empty($favouriteconversationcount) && empty($groupconversationcount);
+
+    // There is an unread conversation somewhere, so that takes priority.
+    if ($unreadcounts['favourites'] > 0 || $unreadcounts['types'][\core_message\api::MESSAGE_CONVERSATION_TYPE_GROUP] > 0 ||
+            $unreadcounts['types'][\core_message\api::MESSAGE_CONVERSATION_TYPE_INDIVIDUAL] > 0) {
+        $favouritesexpanded = $unreadcounts['favourites'] > 0;
+        $groupmessagesexpanded = !$favouritesexpanded &&
+            $unreadcounts['types'][\core_message\api::MESSAGE_CONVERSATION_TYPE_GROUP] > 0;
+        $messagesexpanded = !$groupmessagesexpanded && !$favouritesexpanded &&
+            $unreadcounts['types'][\core_message\api::MESSAGE_CONVERSATION_TYPE_INDIVIDUAL] > 0;
+    }
+
     return $renderer->render_from_template('core_message/message_drawer', [
         'contactrequestcount' => $requestcount,
         'loggedinuser' => [
@@ -877,23 +893,23 @@ function core_message_standard_after_main_region_html() {
         ],
         'overview' => [
             'messages' => [
-                'expanded' => empty($favouriteconversationcount) && empty($groupconversationcount),
+                'expanded' => $messagesexpanded,
                 'count' => [
                     'unread' => $unreadcounts['types'][\core_message\api::MESSAGE_CONVERSATION_TYPE_INDIVIDUAL],
                     'total' => $individualconversationcount
                 ],
                 'placeholders' => array_fill(0, $individualconversationcount, true)
             ],
             'groupmessages' => [
-                'expanded' => empty($favouriteconversationcount) && !empty($groupconversationcount),
+                'expanded' => $groupmessagesexpanded,
                 'count' => [
                     'unread' => $unreadcounts['types'][\core_message\api::MESSAGE_CONVERSATION_TYPE_GROUP],
                     'total' => $groupconversationcount
                 ],
                 'placeholders' => array_fill(0, $groupconversationcount, true)
             ],
             'favourites' => [
-                'expanded' => !empty($favouriteconversationcount),
+                'expanded' => $favouritesexpanded,
                 'count' => [
                     'unread' => $unreadcounts['favourites'],
                     'total' => $favouriteconversationcount