(js) Improve scrolling behavior when deleting msg

Fixes #3489

UI/WebServerResources/js/Mailer/Mailbox.service.js

@@ -192,6 +192,7 @@
 
     if (index >= 0 && index < this.$messages.length) {
       message = this.$messages[index];
+      this.$lastVisibleIndex = Math.max(0, index - 3); // Magic number is NUM_EXTRA from virtual-repeater.js
 
       if (this.$loadMessage(message.uid))
         return message;

UI/WebServerResources/js/Mailer/MessageController.js

@@ -133,7 +133,10 @@
           try {
             if (nextMessage) {
               state.go('mail.account.mailbox.message', { messageId: nextMessage.uid });
-              mailbox.$topIndex = nextIndex;
+              if (nextIndex < mailbox.$topIndex)
+                mailbox.$topIndex = nextIndex;
+              else if (nextIndex > mailbox.$lastVisibleIndex)
+                mailbox.$topIndex = nextIndex - (mailbox.$lastVisibleIndex - mailbox.$topIndex);
             }
             else {
               state.go('mail.account.mailbox');