3 files changed, 59 insertions, 5 deletions

diff --git a/app/controllers/revisions_controller.rb b/app/controllers/revisions_controller.rb
index 4b0c549..0aed243 100644
--- a/app/controllers/revisions_controller.rb
+++ b/app/controllers/revisions_controller.rb
@@ -29,8 +29,13 @@ class RevisionsController < ApplicationController
       redirect_to(node_path(@node)) and return
     end
 
+    @locale_summary  = @end.locale_diff_summary(@start)
+    requested_locale = params[:locale].presence&.to_sym
+    default_locale    = @locale_summary.find { |s| s[:changed] }&.dig(:locale) || I18n.default_locale
+    @locale = @locale_summary.any? { |s| s[:locale] == requested_locale } ? requested_locale : default_locale
+
     @diff_view              = params[:view] == "side_by_side" ? :side_by_side : :inline
-    @diff                   = @end.diff_against(@start, view: @diff_view)
+    @diff                   = @end.diff_against(@start, view: @diff_view, locale: @locale)
     @available_layer_pairs  = @node.available_layer_pairs
     @locked_by_other        = @node.locked? && @node.lock_owner != current_user
   end
diff --git a/app/views/revisions/diff.html.erb b/app/views/revisions/diff.html.erb
index 5638b7a..149b17c 100644
--- a/app/views/revisions/diff.html.erb
+++ b/app/views/revisions/diff.html.erb
@@ -7,20 +7,37 @@
 <p class="diff_comparison_label">
   Comparing <strong><%= describe_page_reference(params[:start_revision]) %></strong>
   against <strong><%= describe_page_reference(params[:end_revision]) %></strong>
+  — showing the <strong><%= @locale.to_s.upcase %></strong> translation
 </p>
 
+<% if @locale_summary.size > 1 %>
+  <p class="diff_locale_toggle">
+    Locale:
+    <% @locale_summary.each_with_index do |s, i| %>
+      <%= " &middot; ".html_safe if i > 0 %>
+      <% label = "#{s[:locale].to_s.upcase}#{' *' if s[:changed]}" %>
+      <% if s[:locale] == @locale %>
+        <strong><%= label %></strong>
+      <% else %>
+        <%= link_to label, diff_node_revisions_path(@node, start_revision: params[:start_revision], end_revision: params[:end_revision], view: @diff_view, locale: s[:locale]) %>
+      <% end %>
+    <% end %>
+  </p>
+  <p class="field_hint">* has a difference between these two versions.</p>
+<% end %>
+
 <p class="diff_view_toggle">
   View:
   <% if @diff_view == :inline %>
     <strong>Inline</strong>
   <% else %>
-    <%= link_to 'Inline', diff_node_revisions_path(@node, start_revision: params[:start_revision], end_revision: params[:end_revision], view: 'inline') %>
+    <%= link_to 'Inline', diff_node_revisions_path(@node, start_revision: params[:start_revision], end_revision: params[:end_revision], view: 'inline', locale: @locale) %>
   <% end %>
   &middot;
   <% if @diff_view == :side_by_side %>
     <strong>Side by side</strong>
   <% else %>
-    <%= link_to 'Side by side', diff_node_revisions_path(@node, start_revision: params[:start_revision], end_revision: params[:end_revision], view: 'side_by_side') %>
+    <%= link_to 'Side by side', diff_node_revisions_path(@node, start_revision: params[:start_revision], end_revision: params[:end_revision], view: 'side_by_side', locale: @locale) %>
   <% end %>
 </p>
 
@@ -31,6 +48,7 @@
     <%= select_tag :start_revision, options_for_select(@node.pages.map{|x| x.revision}, params[:start_revision].to_i) %>
     <%= select_tag :end_revision, options_for_select(@node.pages.map{|x| x.revision}, params[:end_revision].to_i) %>
     <%= hidden_field_tag :view, @diff_view %>
+    <%= hidden_field_tag :locale, @locale %>
     <%= submit_tag 'Diff' %>
   <% end %>
 <% else %>
@@ -44,10 +62,9 @@
       <%= button_to "Diff #{pair.first.to_s.capitalize} vs. #{pair.last.to_s.capitalize}",
             diff_node_revisions_path(@node),
             method: :get,
-            params: { start_revision: pair.first, end_revision: pair.last, view: @diff_view },
+            params: { start_revision: pair.first, end_revision: pair.last, view: @diff_view, locale: @locale },
             form: { class: 'button_to computation' } %>
     <% end %>
-
     <% if !@locked_by_other && (@node.autosave || @node.draft) %>
       <%= button_to revert_node_path(@node), method: :put,
                   params: { return_to: request.fullpath },
diff --git a/test/controllers/revisions_controller_test.rb b/test/controllers/revisions_controller_test.rb
index 76d4e8c..e5726f7 100644
--- a/test/controllers/revisions_controller_test.rb
+++ b/test/controllers/revisions_controller_test.rb
@@ -168,4 +168,36 @@ class RevisionsControllerTest < ActionController::TestCase
     assert_response :success
     assert_select "a[href=?]", node_path(@node)
   end
+
+  test "diff defaults to a locale that actually changed, not the ambient default" do
+    login_as :quentin
+    node = Node.root.children.create!(:slug => "diff_default_locale_test")
+    node.draft.save!
+    node.publish_draft!
+
+    node.find_or_create_draft(@user)
+    Globalize.with_locale(:en) { node.draft.update!(:title => "Changed in English only") }
+    node.draft.save!
+
+    post(:diff, params: { :node_id => node.id, :start_revision => "head", :end_revision => "draft" })
+
+    assert_response :success
+    assert_match(/Changed/, response.body)
+  end
+
+  test "diff respects an explicitly requested locale over the auto-detected one" do
+    login_as :quentin
+    node = Node.root.children.create!(:slug => "diff_explicit_locale_test")
+    node.draft.save!
+    node.publish_draft!
+
+    node.find_or_create_draft(@user)
+    Globalize.with_locale(:en) { node.draft.update!(:title => "English changed") }
+    node.draft.save!
+
+    post(:diff, params: { :node_id => node.id, :start_revision => "head", :end_revision => "draft", :locale => "de" })
+
+    assert_response :success
+    assert_no_match(/English changed/, response.body)
+  end
 end