redmine/app/views/settings/_general.html.erb

<%= form_tag({:action => 'edit'}) do %>

<div class="box tabular settings">
<p><%= setting_text_field :app_title, :size => 30 %></p>

<p><%= setting_text_area :welcome_text, :cols => 60, :rows => 5, :class => 'wiki-edit' %></p>
<%= wikitoolbar_for 'settings_welcome_text' %>


<p><%= setting_text_field :per_page_options, :size => 20 %>
<em class="info"><%= l(:text_comma_separated) %></em></p>

<p><%= setting_text_field :search_results_per_page, :size => 6 %>

<p><%= setting_text_field :activity_days_default, :size => 6 %> <%= l(:label_day_plural) %></p>

<p><%= setting_text_field :host_name, :size => 60 %>
<em class="info"><%= l(:label_example) %>: <%= @guessed_host_and_path %></em></p>

<p><%= setting_select :protocol, [['HTTP', 'http'], ['HTTPS', 'https']] %></p>

<p><%= setting_select :text_formatting, Redmine::WikiFormatting.formats_for_select, :blank => :label_none %>
  <span id="common_mark_info" class="<%= "hidden" unless Setting.text_formatting == "common_mark" %>">
    <label class="block">
      <%= check_box_tag(nil, '', Redmine::Configuration['common_mark_enable_hardbreaks'] == true, disabled: true) %>
      Hardbreaks
    </label>
    <em class="info">
      <%= l(:text_setting_config_change) %>
    </em>
  </span>
</p>

<p><%= setting_check_box :cache_formatted_text %></p>

<p><%= setting_select :wiki_compression, [['Gzip', 'gzip']], :blank => :label_none %></p>

<p><%= setting_text_field :feeds_limit, :size => 6 %></p>

<%= call_hook(:view_settings_general_form) %>
</div>

<%= submit_tag l(:button_save) %>
<% end %>

<%= javascript_tag do %>
  $('#settings_text_formatting').on('change', function(e){
    const formatter = e.target.value;
    const parent_block = document.getElementById("common_mark_info");

    if (formatter == "common_mark") {
      parent_block.classList.remove('hidden');
    } else {
      parent_block.classList.add('hidden');
    }
  });
<% end %>