Index: app/controllers/messages_controller.rb
===================================================================
--- app/controllers/messages_controller.rb	(revision 18419)
+++ app/controllers/messages_controller.rb	(working copy)
@@ -30,22 +30,23 @@
   helper :attachments
   include AttachmentsHelper
 
-  REPLIES_PER_PAGE = 25 unless const_defined?(:REPLIES_PER_PAGE)
-
   # Show a topic and its replies
   def show
     page = params[:page]
     # Find the page of the requested reply
+    replies_order = User.current.wants_comments_in_reverse_order? ? 'DESC' : 'ASC'
+    @replies = @topic.children.
+      reorder("#{Message.table_name}.created_on #{replies_order}, #{Message.table_name}.id #{replies_order}")
+
     if params[:r] && page.nil?
-      offset = @topic.children.where("#{Message.table_name}.id < ?", params[:r].to_i).count
-      page = 1 + offset / REPLIES_PER_PAGE
+      offset = @replies.pluck(:id).index(params[:r].to_i)
+      page = 1 + offset / per_page_option
     end
 
-    @reply_count = @topic.children.count
-    @reply_pages = Paginator.new @reply_count, REPLIES_PER_PAGE, page
-    @replies =  @topic.children.
+    @reply_count = @replies.count
+    @reply_pages = Paginator.new @reply_count, per_page_option, page
+    @replies = @replies.
       includes(:author, :attachments, {:board => :project}).
-      reorder("#{Message.table_name}.created_on ASC, #{Message.table_name}.id ASC").
       limit(@reply_pages.per_page).
       offset(@reply_pages.offset).
       to_a
@@ -107,7 +108,7 @@
     @message.destroy
     flash[:notice] = l(:notice_successful_delete)
     if @message.parent
-      redirect_to board_message_path(@board, @message.parent, :r => r)
+      redirect_to board_message_path(@board, @message.parent)
     else
       redirect_to project_board_path(@project, @board)
     end
Index: app/views/messages/_reply_form.html.erb
===================================================================
--- app/views/messages/_reply_form.html.erb	(nonexistent)
+++ app/views/messages/_reply_form.html.erb	(working copy)
@@ -0,0 +1,14 @@
+<% has_form ||= false %>
+<% if !@topic.locked? && authorize_for('messages', 'reply') %>
+<% if has_form %>
+  <p><%= toggle_link l(:button_reply), "reply", :focus => 'message_content' %></p>
+  <div id="reply" style="display:none;">
+  <%= form_for @reply, :as => :reply, :url => {:action => 'reply', :id => @topic}, :html => {:multipart => true, :id => 'message-form'} do |f| %>
+    <%= render :partial => 'form', :locals => {:f => f, :replying => true} %>
+    <%= submit_tag l(:button_submit) %>
+  <% end %>
+  </div>
+<% else %>
+  <p><%= toggle_link l(:button_reply), "reply", :focus => 'message_content', :scroll => 'message_content' %></p>
+<% end %>
+<% end %>
Index: app/views/messages/show.html.erb
===================================================================
--- app/views/messages/show.html.erb	(revision 18419)
+++ app/views/messages/show.html.erb	(working copy)
@@ -36,9 +36,7 @@
 <% unless @replies.empty? %>
 <div id="replies">
 <h3 class="comments icon icon-comments"><%= l(:label_reply_plural) %> (<%= @reply_count %>)</h3>
-<% if !@topic.locked? && authorize_for('messages', 'reply') && @replies.size >= 3 %>
-  <p><%= toggle_link l(:button_reply), "reply", :focus => 'message_content', :scroll => "message_content" %></p>
-<% end %>
+<%= render :partial => 'reply_form', :locals => {:has_form => User.current.wants_comments_in_reverse_order? } %>
 <% @replies.each do |message| %>
   <div class="message reply" id="<%= "message-#{message.id}" %>">
     <div class="contextual">
@@ -76,17 +74,9 @@
   </div>
 <% end %>
 </div>
-<span class="pagination"><%= pagination_links_full @reply_pages, @reply_count, :per_page_links => false %></span>
+<span class="pagination"><%= pagination_links_full @reply_pages, @reply_count %></span>
 <% end %>
 
-<% if !@topic.locked? && authorize_for('messages', 'reply') %>
-<p><%= toggle_link l(:button_reply), "reply", :focus => 'message_content' %></p>
-<div id="reply" style="display:none;">
-<%= form_for @reply, :as => :reply, :url => {:action => 'reply', :id => @topic}, :html => {:multipart => true, :id => 'message-form'} do |f| %>
-  <%= render :partial => 'form', :locals => {:f => f, :replying => true} %>
-  <%= submit_tag l(:button_submit) %>
-<% end %>
-</div>
-<% end %>
+<%= render :partial => 'reply_form', :locals => {:has_form => @replies.empty? || !User.current.wants_comments_in_reverse_order? } %>
 
 <% html_title @topic.subject %>
Index: test/functional/messages_controller_test.rb
===================================================================
--- test/functional/messages_controller_test.rb	(revision 18419)
+++ test/functional/messages_controller_test.rb	(working copy)
@@ -252,7 +252,7 @@
           :id => 2
         }
     end
-    assert_redirected_to '/boards/1/topics/1?r=2'
+    assert_redirected_to '/boards/1/topics/1'
     assert_equal I18n.t(:notice_successful_delete), flash[:notice]
     assert_nil Message.find_by_id(2)
   end
Index: test/integration/messages_test.rb
===================================================================
--- test/integration/messages_test.rb	(nonexistent)
+++ test/integration/messages_test.rb	(working copy)
@@ -0,0 +1,107 @@
+# frozen_string_literal: true
+
+# Redmine - project management software
+# Copyright (C) 2006-2019  Jean-Philippe Lang
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+
+require File.expand_path('../../test_helper', __FILE__)
+
+class MessagesTest < Redmine::IntegrationTest
+  fixtures :projects, :users, :email_addresses, :user_preferences, :members,
+           :member_roles, :roles, :boards, :messages, :enabled_modules
+
+  def setup
+    message = Message.find(1)
+    assert_difference 'Message.count', 60 do
+      60.times do
+        message.children << Message.new(
+          :subject => 'Reply',
+          :content => 'Reply body',
+          :author_id => 2,
+          :board_id => 1)
+      end
+    end
+    @reply_ids = message.children.map(&:id).sort
+    @per_page = (@reply_ids.count - 1).to_s
+
+    log_user('jsmith', 'jsmith')
+  end
+
+  def test_show_with_pagination_should_ascending_order
+    with_settings :per_page_options => @per_page do
+      put '/my/account', :params => { :pref => { :comments_sorting => 'asc' }}
+      assert !User.current.wants_comments_in_reverse_order?
+
+      get '/boards/1/topics/1', :params => { :r => @reply_ids.last }
+      assert_select 'span.pagination > ul.pages > li.current > span', text: '2'
+    end
+  end
+
+  def test_show_with_pagination_should_descending_order
+    with_settings :per_page_options => @per_page do
+      put '/my/account', :params => { :pref => { :comments_sorting => 'desc' }}
+      assert User.current.wants_comments_in_reverse_order?
+
+      get '/boards/1/topics/1', :params => { :r => @reply_ids.last }
+      assert_select 'span.pagination > ul.pages > li.current > span', text: '1'
+    end
+  end
+
+end
+require File.expand_path('../../test_helper', __FILE__)
+
+class MessagesTest < Redmine::IntegrationTest
+  fixtures :projects, :users, :email_addresses, :user_preferences, :members,
+           :member_roles, :roles, :boards, :messages, :enabled_modules
+
+  def setup
+    message = Message.find(1)
+    assert_difference 'Message.count', 60 do
+      60.times do
+        message.children << Message.new(
+          :subject => 'Reply',
+          :content => 'Reply body',
+          :author_id => 2,
+          :board_id => 1)
+      end
+    end
+    @reply_ids = message.children.map(&:id).sort
+    @per_page = (@reply_ids.count - 1).to_s
+
+    log_user('jsmith', 'jsmith')
+  end
+
+  def test_show_with_pagination_should_ascending_order
+    with_settings :per_page_options => @per_page do
+      put '/my/account', :params => { :pref => { :comments_sorting => 'asc' }}
+      assert !User.current.wants_comments_in_reverse_order?
+
+      get '/boards/1/topics/1', :params => { :r => @reply_ids.last }
+      assert_select 'span.pagination > ul.pages > li.current > span', text: '2'
+    end
+  end
+
+  def test_show_with_pagination_should_descending_order
+    with_settings :per_page_options => @per_page do
+      put '/my/account', :params => { :pref => { :comments_sorting => 'desc' }}
+      assert User.current.wants_comments_in_reverse_order?
+
+      get '/boards/1/topics/1', :params => { :r => @reply_ids.last }
+      assert_select 'span.pagination > ul.pages > li.current > span', text: '1'
+    end
+  end
+
+end