Thank you for posting the patch.
But I think it would be better not to assume that the value of REPLIES_PER_PAGE constant is always 25. The test in your patch may fail if #26033 is implemented.
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Hello, Go MAEDA. Thank you for your advice.
I improved it in order of reply display considering "per_page".
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The patch works fine for me. And it also implements #26033.
Setting target version to 3.5.0.
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This patch is great work but sometimes did not work well on my environment.

my environment:
* MySQL 5.6.42 (ClearDB)
* Ruby 2.5.5p157
* Redmine 4.0.3 + reply_display_order_with_pre_page.patch

When showing the topic where
* count of the topic children is larger than @per_page_option@, and
* some children has more than one attachment files

In which case, @r=xx@ parameter may not be expanded correctly due to incorrect paging.

On my environment, @@replies.pluck(:id)@ generated an array containing duplicate IDs.
I could not identify the root cause, but I found a line @includes(:author, :attachments, {:board => :project})@ cause this problem.
Therefore, I corrected this problem as follows to avoid this problem:

<pre><code class="diff">
--- a/app/controllers/messages_controller.rb
+++ b/app/controllers/messages_controller.rb
@@ -34,7 +34,6 @@ class MessagesController < ApplicationController
# Find the page of the requested reply
replies_order = User.current.wants_comments_in_reverse_order? ? 'DESC' : 'ASC'
@replies = @topic.children.
- includes(:author, :attachments, {:board => :project}).
reorder("#{Message.table_name}.created_on #{replies_order}, #{Message.table_name}.id #{replies_order}")

if params[:r] && page.nil?
@@ -45,6 +44,7 @@ class MessagesController < ApplicationController
@reply_count = @replies.count
@reply_pages = Paginator.new @reply_count, per_page_option, page
@replies = @replies.
+ includes(:author, :attachments, {:board => :project}).
limit(@reply_pages.per_page).
offset(@reply_pages.offset).
to_a
</code></pre>

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I improved Minoru-Maeda-san's patch. My patch includes:

* This issue #26030 itself;
* display considering "per_page" (#26033);
* place "reply" block considering the display order (like #31438);
* fix regression that I reported in #26030#note-6;
* fix regression where redirection after deleting a message raises an error; and
* fix test codes to pass @rails test -n /Message.*Test/@.
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Popoki Tom (@cat_in_136) wrote:
> I improved Minoru-Maeda-san's patch. My patch includes:
>
> * This issue #26030 itself;
> * display considering "per_page" (#26033);
> * place "reply" block considering the display order (like #31438);
> * fix regression that I reported in #26030#note-6;
> * fix regression where redirection after deleting a message raises an error; and
> * fix test codes to pass @rails test -n /Message.*Test/@.

Thanks for updating the patch. Can you check the content of @test/integration/messages_test.rb@? It seems the class @class MessagesTest@ is duplicated.

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Opps, Thank you for checking my patch. I've removed the duplicated @MessageTest@.

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Popoki Tom (@cat_in_136) wrote:
> Opps, Thank you for checking my patch. I've removed the duplicated @MessageTest@.

My patch has still a some degrade bug where @r=xx@ parameter may not be expanded correctly. This patch should not be merged.
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4th revision of the patch attached.

I've rewrite MessageController#show to fix degrade bug where `r=xx` parameter may not be expanded correctly.
My internal testing (dogfooding) is on-going on my private redmine instance.
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relates,Closed,26033,Respect "Objects per page" option when displaying forum replies
relates,New,11120,Order replies of messages boards based on user preference