Views.Thread: Fix scrolling to root post

src/Views/Thread.vala

@@ -61,7 +61,7 @@ public class Tootle.Views.Thread : Views.Base, IAccountListener {
                 on_content_changed ();
 
                 int x,y;
-                translate_coordinates (root_widget, 0, 0, out x, out y);
+                translate_coordinates (root_widget, 0, header.get_allocated_height (), out x, out y);
                 scrolled.vadjustment.value = (double)(y*-1);
             })
             .exec ();