Jump to furst unread message when selecting a conversation: Closes #6243

Although a duplicate of #4639, where we decided not to implement this
feature in lieu of implementing #5137 (compress collapsed read messages),
since we don't know when that will be ready, this is a good stop-gap,
as current behavior is annoying with long conversations.  Andrea's
code is cleanly delineated from the other code, and should be easily
removed if/when the time comes.

src/client/geary-controller.vala

@@ -549,6 +549,7 @@ public class GearyController {
             main_window.conversation_viewer.add_message(email);
         
         main_window.conversation_viewer.unhide_last_email();
+        main_window.conversation_viewer.show_first_visible_email();
     }
     
     private void on_show_message_completed(Object? source, AsyncResult result) {

src/client/views/conversation-viewer.vala

@@ -339,6 +339,25 @@ public class ConversationViewer : Gtk.Box {
             }
         }
     }
+
+    public void show_first_visible_email() {
+        // Select the first email element that does not have "hide" in its
+        // class list. The constraint on the id is required because there
+        // are some elements (like #selection_counter) that have the "email"
+        // class, but that are not messages.
+        WebKit.DOM.HTMLElement first_visible_email =
+            Util.DOM.select(web_view.get_dom_document(), ".email[id^=message_]:not(.hide)");
+        if (first_visible_email != null) {
+            // Select the sibling which, if it exists, is the previous hidden
+            // message. This is a way Geary has to say "hey, this conversation
+            // is not new: there are some messages above".
+            WebKit.DOM.HTMLElement first_visible_email_sibling = 
+                (WebKit.DOM.HTMLElement) first_visible_email.previous_sibling;
+            if (first_visible_email_sibling != null) {
+                web_view.scroll_to_element(first_visible_email_sibling);
+            }
+        }
+    }
     
     private Geary.Email? get_email_from_element(WebKit.DOM.Element element) {
         // First get the email container.

src/client/views/conversation-web-view.vala

@@ -313,5 +313,9 @@ public class ConversationWebView : WebKit.WebView {
     public WebKit.DOM.HTMLDivElement create_div() throws Error {
         return get_dom_document().create_element("div") as WebKit.DOM.HTMLDivElement;
     }
+
+    public void scroll_to_element(WebKit.DOM.HTMLElement element) {
+        get_dom_document().get_default_view().scroll(element.offset_left, element.offset_top);
+    }
 }