capture doesn't work outside of the view it's in?

another_partial.html.erb

alert('something');

application_helper.rb

def broken_helper
  render_to_string('another_partial', format: false)
end

def working_helper
  %{alert('something');}.html_safe
end

bad_capture.html.erb

<!-- view template -->
<% @myjs = capture do %>
  <%= broken_helper %>
<% end %>
<%= @myjs %>
<!-- output: "something" -->

<!-- alternate approach -->
<% content_for(:javascript) do %>
  <%= broken_helper %>
<% end %>

<!-- working approach -->
<% content_for(:js2) do %>
  <%= working_helper %>
<% end %>

layout.html.erb

<!-- layout template -->
<script><%= @myjs %></script>
<!-- output: "" -->

<script><%= yield(:javascript) %></script>
<!-- output: "" -->

<script><%= yield(:js2) %></script>
<!-- output: "alert('something');" -->

built this out into a test repo: coldnebo/rails_content_issue@3f4b135#diff-c41b830814781a1cd840bfed7a09e6b8R15

confirmed that the problem is in Rails3, but fixed in Rails4. see https://github.com/coldnebo/rails_content_issue/blob/master/README.md