11 files changed, 97 insertions, 70 deletions

diff --git a/app/controllers/revisions_controller.rb b/app/controllers/revisions_controller.rb
index cddb9e72..b8e66218 100644
--- a/app/controllers/revisions_controller.rb
+++ b/app/controllers/revisions_controller.rb
@@ -8,9 +8,9 @@ class RevisionsController < ApplicationController
   layout 'admin'
 
   def index
-    @node   = Node.find(params[:node_id])
-    @pages  = @node.pages.all
-    @locale = resolve_locale(params[:locale])
+    @node               = Node.find(params[:node_id])
+    @pages              = @node.pages.all
+    @translation_locale = resolve_locale(params[:translation_locale])
   end
 
   def diff
@@ -31,21 +31,21 @@ 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
+    @locale_summary     = @end.locale_diff_summary(@start)
+    requested_locale    = params[:translation_locale].presence&.to_sym
+    default_locale      = @locale_summary.find { |s| s[:changed] }&.dig(:locale) || I18n.default_locale
+    @translation_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, locale: @locale)
+    @diff                   = @end.diff_against(@start, view: @diff_view, locale: @translation_locale)
     @available_layer_pairs  = @node.available_layer_pairs
     @locked_by_other        = @node.locked? && @node.lock_owner != current_user
   end
 
   def show
-    @node     = Node.find(params[:node_id])
-    @page     = @node.pages.find(params[:id])
-    @locale   = resolve_locale(params[:locale])
+    @node               = Node.find(params[:node_id])
+    @page               = @node.pages.find(params[:id])
+    @translation_locale = resolve_locale(params[:translation_locale])
   end
 
   def restore
diff --git a/app/helpers/revisions_helper.rb b/app/helpers/revisions_helper.rb
index a6290133..25a2aa38 100644
--- a/app/helpers/revisions_helper.rb
+++ b/app/helpers/revisions_helper.rb
@@ -4,4 +4,21 @@ module RevisionsHelper
   def describe_page_reference(ref)
     %w[head draft autosave].include?(ref.to_s) ? ref.to_s.capitalize : "revision #{ref}"
   end
+
+  def neighbour_revision_pairs node, start_ref, end_ref
+    return [nil, nil] unless start_ref.to_s.match?(/\A\d+\z/) &&
+                             end_ref.to_s.match?(/\A\d+\z/)
+
+    a, b = start_ref.to_i, end_ref.to_i
+    return [nil, nil] unless (a - b).abs == 1
+
+    lo, hi  = [a, b].minmax
+    forward = a < b
+    earlier = forward ? [lo - 1, lo] : [lo, lo - 1]
+    later   = forward ? [hi, hi + 1] : [hi + 1, hi]
+
+    [earlier, later].map do |pair|
+      pair if pair.all? { |rev| node.pages.exists?(:revision => rev) }
+    end
+  end
 end
diff --git a/app/views/layouts/_nav_controls.html.erb b/app/views/layouts/_nav_controls.html.erb
index bae1082b..70d5130c 100644
--- a/app/views/layouts/_nav_controls.html.erb
+++ b/app/views/layouts/_nav_controls.html.erb
@@ -9,7 +9,8 @@
 <% end %>
 
 <span class="nav_group">
-  <%= link_to url_for(locale: (:en unless I18n.locale == :en)),
+  <% locale_target = request.query_parameters.symbolize_keys.except(:controller, :action, :only_path, :host, :port, :protocol, :anchor, :script_name).merge(:locale => (:en unless I18n.locale == :en)) %>
+  <%= link_to url_for(locale_target),
         class: "nav_toggle nav_locale",
         "aria-label" => t(".switch_locale"), title: t(".switch_locale") do %>
     <%= I18n.locale == :en ? "DE" : "EN" %>
diff --git a/app/views/node_actions/_change_details.html.erb b/app/views/node_actions/_change_details.html.erb
index 19e2fe29..2da8bfd5 100644
--- a/app/views/node_actions/_change_details.html.erb
+++ b/app/views/node_actions/_change_details.html.erb
@@ -25,9 +25,9 @@
           <br/>
           <% if action_entry.page && action_entry.node %>
             <% if (diff_params = action_entry.diff_link_params) %>
-              <%= link_to t("node_actions.view_diff"), diff_node_revisions_path(action_entry.node, diff_params.merge(:locale => locale)) %>
+              <%= link_to t("node_actions.view_diff"), diff_node_revisions_path(action_entry.node, diff_params.merge(:translation_locale => locale)) %>
             <% else %>
-              <%= link_to t("node_actions.view_revision"), node_revision_path(action_entry.node, action_entry.page, :locale => locale) %>
+              <%= link_to t("node_actions.view_revision"), node_revision_path(action_entry.node, action_entry.page, :translation_locale => locale) %>
             <% end %>
           <% end %>
         </td>
diff --git a/app/views/page_translations/edit.html.erb b/app/views/page_translations/edit.html.erb
index f398a544..fb879568 100644
--- a/app/views/page_translations/edit.html.erb
+++ b/app/views/page_translations/edit.html.erb
@@ -2,7 +2,7 @@
   <%= t(".editing_prefix") %> <strong><%= @locale.to_s.upcase %></strong> <%= t(".translation_of") %> <strong><%= title_for_node(@node) %></strong>.
 </div>
 
-<p class="node_action_bar standalone_action_bar"><%= link_to t("page_translations.revision_history_link"), node_revisions_path(@node, :locale => @locale) %></p>
+<p class="node_action_bar standalone_action_bar"><%= link_to t("page_translations.revision_history_link"), node_revisions_path(@node, :translation_locale => @locale) %></p>
 
 <p class="field_hint">
   <%= t(".metadata_hint_prefix") %> <%= link_to t(".metadata_hint_link"), edit_node_path(@node) %>.
diff --git a/app/views/page_translations/show.html.erb b/app/views/page_translations/show.html.erb
index 0fbcd0b6..5f94ac38 100644
--- a/app/views/page_translations/show.html.erb
+++ b/app/views/page_translations/show.html.erb
@@ -1,6 +1,6 @@
 <h1><%= t(".title") %> — <%= title_for_node(@node) %></h1>
 
-<p class="node_action_bar standalone_action_bar"><%= link_to t("page_translations.revision_history_link"), node_revisions_path(@node, :locale => @locale) %></p>
+<p class="node_action_bar standalone_action_bar"><%= link_to t("page_translations.revision_history_link"), node_revisions_path(@node, :translation_locale => @locale) %></p>
 
 <div class="translation_compare">
   <div class="translation_compare_column">
diff --git a/app/views/revisions/diff.html.erb b/app/views/revisions/diff.html.erb
index 1318bd37..ba320365 100644
--- a/app/views/revisions/diff.html.erb
+++ b/app/views/revisions/diff.html.erb
@@ -4,57 +4,51 @@
   <%= link_to t(".revisions_link"), node_revisions_path(@node) %>
 </p>
 
-<p class="diff_comparison_label">
-  <%= t(".comparing") %> <strong><%= describe_page_reference(params[:start_revision]) %></strong>
-  <%= t(".against") %> <strong><%= describe_page_reference(params[:end_revision]) %></strong>
-  <%= t(".showing_prefix") %> <strong><%= @locale.to_s.upcase %></strong> <%= t(".translation_suffix") %>
-</p>
+<% numeric_comparison = params[:start_revision].to_s =~ /\A\d+\z/ && params[:end_revision].to_s =~ /\A\d+\z/ %>
+<% start_control = numeric_comparison ?
+     select_tag(:start_revision, options_for_select(@node.pages.map(&:revision), params[:start_revision].to_i)) :
+     content_tag(:strong, describe_page_reference(params[:start_revision])) + hidden_field_tag(:start_revision, params[:start_revision]) %>
+<% end_control = numeric_comparison ?
+     select_tag(:end_revision, options_for_select(@node.pages.map(&:revision), params[:end_revision].to_i)) :
+     content_tag(:strong, describe_page_reference(params[:end_revision])) + hidden_field_tag(:end_revision, params[:end_revision]) %>
+<% locale_control = select_tag(:translation_locale, options_for_select(
+     @locale_summary.map { |s| [s[:changed] ? t("revisions.locale_changed", :lang => s[:locale].to_s.upcase) : s[:locale].to_s.upcase, s[:locale]] },
+     @translation_locale)) %>
+<% view_control = select_tag(:view, options_for_select(
+     [[t("revisions.inline"), "inline"], [t("revisions.side_by_side"), "side_by_side"]], @diff_view.to_s)) %>
+<% earlier, later = neighbour_revision_pairs(@node, params[:start_revision], params[:end_revision]) %>
 
-<% if @locale_summary.size > 1 %>
-  <p class="diff_locale_toggle">
-    <%= t("revisions.locale_label") %>
-    <% @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 %>
+<%= form_tag diff_node_revisions_path(@node), :method => :get, :class => "diff_controls" do %>
+  <p class="diff_comparison_label">
+    <%= t(".comparing_html", :start => start_control, :end => end_control,
+          :lang => locale_control, :view => view_control) %>
+  </p>
+  <p class="node_action_bar standalone_action_bar diff_step_bar">
+    <% if earlier %>
+      <%= link_to t(".pair_link", :from => earlier.first, :to => earlier.last), diff_node_revisions_path(@node,
+            start_revision: earlier.first, end_revision: earlier.last,
+            view: @diff_view, translation_locale: @translation_locale) %>
+    <% else %>
+      <span class="disabled_action">&mdash;</span>
+    <% end %>
+    <%= submit_tag t(".diff_button") %>
+    <% if later %>
+      <%= link_to t(".pair_link", :from => later.first, :to => later.last), diff_node_revisions_path(@node,
+            start_revision: later.first, end_revision: later.last,
+            view: @diff_view, translation_locale: @translation_locale) %>
+    <% else %>
+      <span class="disabled_action">&mdash;</span>
     <% end %>
   </p>
-  <p class="field_hint"><%= t(".changed_hint") %></p>
 <% end %>
 
-<p class="diff_view_toggle">
-  <%= t(".view_label") %>
-  <% if @diff_view == :inline %>
-    <strong><%= t("revisions.inline") %></strong>
-  <% else %>
-    <%= link_to t("revisions.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><%= t("revisions.side_by_side") %></strong>
-  <% else %>
-    <%= link_to t("revisions.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>
-
-<% numeric_comparison = params[:start_revision].to_s =~ /\A\d+\z/ && params[:end_revision].to_s =~ /\A\d+\z/ %>
-
-<% if numeric_comparison %>
-  <%= form_tag diff_node_revisions_path do %>
-    <%= 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 t(".diff_button") %>
-  <% end %>
-<% else %>
-  <p class="node_action_bar standalone_action_bar"><%= link_to t(".compare_numbered"), node_revisions_path(@node) %></p>
+<% unless numeric_comparison %>
+  <p class="node_action_bar standalone_action_bar">
+    <%= link_to t(".compare_numbered"), node_revisions_path(@node) %>
+  </p>
 <% end %>
 
+
 <% if @available_layer_pairs.present? %>
   <div class="node_action_bar standalone_action_bar">
     <% @available_layer_pairs.each do |pair| %>
@@ -62,7 +56,7 @@
       <%= button_to t("nodes.show.diff_layers", :from => t("nodes.show.layer_#{pair.first}"), :to => t("nodes.show.layer_#{pair.last}")),
             diff_node_revisions_path(@node),
             method: :get,
-            params: { start_revision: pair.first, end_revision: pair.last, view: @diff_view, locale: @locale },
+            params: { start_revision: pair.first, end_revision: pair.last, view: @diff_view, translation_locale: @translation_locale },
             form: { class: 'button_to computation' } %>
     <% end %>
     <% if !@locked_by_other && (@node.autosave || @node.draft) %>
diff --git a/app/views/revisions/index.html.erb b/app/views/revisions/index.html.erb
index 6bcf1820..d118f96a 100644
--- a/app/views/revisions/index.html.erb
+++ b/app/views/revisions/index.html.erb
@@ -9,10 +9,10 @@
     <%= t("revisions.locale_label") %>
     <% ([I18n.default_locale] + Page.non_default_locales).each_with_index do |locale, i| %>
       <%= " &middot; ".html_safe if i > 0 %>
-      <% if locale == @locale %>
+      <% if locale == @translation_locale %>
         <strong><%= locale.to_s.upcase %></strong>
       <% else %>
-        <%= link_to locale.to_s.upcase, node_revisions_path(@node, :locale => locale) %>
+        <%= link_to locale.to_s.upcase, node_revisions_path(@node, :translation_locale => locale) %>
       <% end %>
     <% end %>
   </p>
@@ -34,7 +34,7 @@
       <td colspan="8">
         <%= button_to t(".diff_revisions"), diff_node_revisions_path(@node),
               method: :post,
-              params: { locale: @locale },
+              params: { translation_locale: @translation_locale },
               form: { id: 'diff_form', class: 'button_to computation' },
               disabled: true %>
         <span id="diff_selection_label" data-against="<%= t(".against_word") %>" data-selected="<%= t(".selected_word") %>"></span>
@@ -50,10 +50,10 @@
       <td><%= radio_button_tag :start_revision, page.revision, index == 1 %></td>
       <td><%= radio_button_tag :end_revision, page.revision, index == 0 %></td>
       <td class="revision"><%= page.revision %></td>
-      <td class="title"><%= page.translations.find_by(:locale => @locale)&.title || "—" %></td>
+      <td class="title"><%= page.translations.find_by(:locale => @translation_locale)&.title || "—" %></td>
       <td class="user"><%= page.editor.try(:login) %></td>
       <td class="date"><%= page.updated_at %></td>
-      <td><%= link_to t(".show_link"), node_revision_path(@node, page, :locale => @locale) %></td>
+      <td><%= link_to t(".show_link"), node_revision_path(@node, page, :translation_locale => @translation_locale) %></td>
       <td>
         <%= button_to t(".restore_link"), restore_node_revision_path(@node, page),
               method: :put,
diff --git a/app/views/revisions/show.html.erb b/app/views/revisions/show.html.erb
index a39071c6..addcfa1a 100644
--- a/app/views/revisions/show.html.erb
+++ b/app/views/revisions/show.html.erb
@@ -1,13 +1,13 @@
-<% translation = @page.translations.find_by(:locale => @locale) %>
+<% translation = @page.translations.find_by(:locale => @translation_locale) %>
 <div id="admin_layout" class="show_node">
-  <h1><%= t(".title", :rev => @page.revision) %>: <%= translation&.title %> <small>(<%= @locale.to_s.upcase %>)</small></h1>
+  <h1><%= t(".title", :rev => @page.revision) %>: <%= translation&.title %> <small>(<%= @translation_locale.to_s.upcase %>)</small></h1>
 
   <div id="content">
     <div class="layout_row_label"><%= t("admin.columns.actions") %></div>
     <div class="layout_row_content info_group">
       <div class="info_group_items">
         <div class="info_item">
-          <%= link_to t(".show_all"), node_revisions_path(@node, :locale => @locale) %>
+          <%= link_to t(".show_all"), node_revisions_path(@node, :translation_locale => @translation_locale) %>
         </div>
       </div>
     </div>
diff --git a/public/stylesheets/admin.css b/public/stylesheets/admin.css
index f415379a..39badaa6 100644
--- a/public/stylesheets/admin.css
+++ b/public/stylesheets/admin.css
@@ -1019,6 +1019,19 @@ div.layout_row_content {
   height: 1.25rem;
 }
 
+.diff_step_bar .disabled_action {
+  border: none;
+  padding: 0;
+  font-weight: normal;
+}
+
+.diff_step_bar > a,
+.diff_step_bar .disabled_action {
+  display: inline-flex;
+  justify-content: center;
+  min-width: 8ch;
+}
+
 .info_group_items {
   display: flex;
   flex-wrap: wrap;
diff --git a/test/controllers/revisions_controller_test.rb b/test/controllers/revisions_controller_test.rb
index 8311735c..34f00a4f 100644
--- a/test/controllers/revisions_controller_test.rb
+++ b/test/controllers/revisions_controller_test.rb
@@ -146,7 +146,9 @@ class RevisionsControllerTest < ActionController::TestCase
 
     post(:diff, params: { :node_id => @node.id, :start_revision => "head", :end_revision => "draft" })
     assert_response :success
-    assert_select "a", I18n.t("revisions.side_by_side")
+    assert_select "select[name='view'] option", :text => I18n.t("revisions.side_by_side")
+    assert_select "input[type='hidden'][name='start_revision'][value='head']"
+    assert_select "input[type='hidden'][name='end_revision'][value='draft']"
   end
 
   test "diffing two revisions also shows tag, template, and asset changes" do