Make revision diffs locale-aware, defaulting to whichever locale changed

diff_against compared title/abstract/body under whatever I18n.locale
happened to be ambient, with no concept of 'diff this translation
specifically' -- so a change confined to one locale was invisible to
Diff Head vs. Draft regardless of which locale you were looking at
when you clicked it, exactly the 'yields nothing' complaint from
earlier this session.

Page#diff_against gains a locale: keyword, additive only -- nil
preserves the exact original ambient-locale behavior every existing
caller and test already depends on; passing a locale switches to
reading each side's actual PageTranslation row directly, same
fallback-free reasoning as Page#translation_summary. Page#locale_diff_summary
reports one entry per locale present on either side, so an added or
removed translation counts as a change even where content matches
everywhere it exists on both.

RevisionsController#diff now resolves a real locale before diffing --
defaulting to whichever locale actually changed, falling back to the
default locale only when nothing did -- and the view carries that
locale through every existing control (view toggle, layer-pair
buttons, the revision-select form) so it and the view/layer-pair axis
stay independently selectable rather than resetting each other.

2 files changed, 27 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 },