If I insert code:

<pre>
<% content_for :header_tags do %>
<%= stylesheet_link_tag :issue_hot_buttons_sidebar, :plugin => 'issue_hot_buttons_plugin' %>
<% end %>
</pre>

in redmine view near hook, it work fine, but in the hook It doesn't work.
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Please, explain what you're doing exactly (eg. post the code of your hook class, or at least give the hook name).
Using @<% content_for :header_tags do %>@ in a plugin view actually works as expected.
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For example. init.rb code:

<pre>
require 'redmine'

class Hooks < Redmine::Hook::ViewListener
render_on :view_issues_show_details_bottom, :partial => 'assets', :layout => false
end

Redmine::Plugin.register :issue_hot_buttons do
name 'Issue Hot Buttons Plugin'
author 'Mike Kolganov, Thumbtack Inc.'
description 'Plugin for Redmine that add buttons for often used actions to issue page'
version '0.4.1'
url 'https://github.com/mikekolganov/redmine-issue-hot-buttons'
end
</pre>

app/views/issues/_assets.html.erb content is:

<pre>
<% content_for :header_tags do %>
<%= javascript_include_tag 'hot_buttons.js', :plugin => 'issue_hot_buttons' %>
<%= stylesheet_link_tag 'hot_buttons.css', :plugin => 'issue_hot_buttons' %>
<% end %>

Test Test Test!
</pre>

If I open any issue, I see "Test Test Test!", but links to "hot_buttons.js" and "hot_buttons.css" are absent.

If I modify code in app/views/issues/_assets.html.erb:

<pre>
<%= javascript_include_tag 'hot_buttons.js', :plugin => 'issue_hot_buttons' %>
<%= stylesheet_link_tag 'hot_buttons.css', :plugin => 'issue_hot_buttons' %>
</pre>

Links appear inside tag "body":
<pre>
<link href="/plugin_assets/issue_hot_buttons/stylesheets/hot_buttons.css?1339038118" media="screen" rel="stylesheet" type="text/css">
<script src="/plugin_assets/issue_hot_buttons/javascripts/hot_buttons.js?1339107838" type="text/javascript"></script>
</pre>

I would like include these links in tag "head"
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Fixed in r9785.
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Merged in 2.0-stable.
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relates,Needs feedback,13456,<% content_for :header_tags do %> doesn't work inside hook
relates,Needs feedback,11527,content_for :header_tag not working within partials