Select revisions to compare by picking rows

Two hidden fields hold the pair, picking a row pushes it as the target and
the previous target becomes the source, so any pair takes two picks. Source
and target carry the diff view's own colours, in the rows and in the sticky
bar's readout.

Rows are focusable and respond to Enter and Space, the readout is the
accessible equivalent of the colour, so it is aria-live.

4 files changed, 96 insertions, 55 deletions

diff --git a/app/views/revisions/index.html.erb b/app/views/revisions/index.html.erb
index c913b2a9..e951016f 100644
--- a/app/views/revisions/index.html.erb
+++ b/app/views/revisions/index.html.erb
@@ -18,11 +18,11 @@
   </p>
 <% end %>
 
+<% pages = (@pages || @node.pages.all).reverse %>
+
 <table id="revisions" class="admin_table revisions_table">
   <thead>
     <tr class="header">
-      <th><%= t(".first") %></th>
-      <th><%= t(".last") %></th>
       <th><%= t("admin.columns.rev") %></th>
       <th><%= t("admin.columns.title") %></th>
       <th><%= t("admin.columns.editor") %></th>
@@ -31,24 +31,27 @@
       <th></th>
     </tr>
     <tr class="diff_sticky_bar">
-      <td colspan="8">
-        <%= button_to t(".diff_revisions"), diff_node_revisions_path(@node),
-              method: :post,
-              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>
-        <label><%= radio_button_tag :view, 'inline', true %> <%= t("revisions.inline") %></label>
-        <label><%= radio_button_tag :view, 'side_by_side', false %> <%= t("revisions.side_by_side") %></label>
+      <td colspan="6">
+        <%= form_tag diff_node_revisions_path(@node), :method => :post,
+              :id => "diff_form", :class => "button_to computation" do %>
+          <%= hidden_field_tag :translation_locale, @translation_locale %>
+          <%= hidden_field_tag :start_revision, pages[1]&.revision %>
+          <%= hidden_field_tag :end_revision,   pages[0]&.revision %>
+          <%= submit_tag t(".diff_revisions") %>
+          <span id="diff_selection_label" aria-live="polite">
+            <%= t(".diff_selection_html",
+                  :from => tag.span(:id => "diff_from"),
+                  :to   => tag.span(:id => "diff_to")) %>
+          </span>
+          <label><%= radio_button_tag :view, 'inline', true %> <%= t("revisions.inline") %></label>
+          <label><%= radio_button_tag :view, 'side_by_side', false %> <%= t("revisions.side_by_side") %></label>
+        <% end %>
       </td>
     </tr>
   </thead>
   <tbody>
-  <% pages = (@pages || @node.pages.all).reverse %>
-  <% pages.each_with_index do |page, index| %>
-    <tr>
-      <td><%= radio_button_tag :start_revision, page.revision, index == 1 %></td>
-      <td><%= radio_button_tag :end_revision, page.revision, index == 0 %></td>
+  <% pages.each do |page| %>
+    <tr data-revision="<%= page.revision %>" tabindex="0" aria-pressed="false">
       <td class="revision"><%= page.revision %></td>
       <td class="title"><%= page.translations.find_by(:locale => @translation_locale)&.title || "—" %></td>
       <td class="user"><%= page.editor.try(:login) %></td>
@@ -65,45 +68,63 @@
 </table>
 
 <%= javascript_tag nonce: true do %>
-  function update_diff_button_state() {
-    var start = document.querySelector('input[name="start_revision"]:checked');
-    var end = document.querySelector('input[name="end_revision"]:checked');
-    var valid = start && end && start.value !== end.value;
-    document.querySelector('#diff_form button[type="submit"]').disabled = !valid;
+  (function () {
+    var table = document.getElementById('revisions');
+    var form  = document.getElementById('diff_form');
+    if (!table || !form) { return; }
 
-    var label = document.getElementById('diff_selection_label');
-    if (start && end) {
-      label.textContent = start.value + ' ' + label.dataset.against + ' ' + end.value;
-    } else if (start || end) {
-      label.textContent = (start || end).value + ' ' + label.dataset.selected;
-    } else {
-      label.textContent = '';
-    }
-  }
+    var from_field = form.querySelector('input[name="start_revision"]');
+    var to_field   = form.querySelector('input[name="end_revision"]');
+    var from_label = document.getElementById('diff_from');
+    var to_label   = document.getElementById('diff_to');
+    var readout    = document.getElementById('diff_selection_label');
+    var submit     = form.querySelector('input[type="submit"]');
+    var rows       = table.querySelectorAll('tbody tr');
 
-  document.querySelectorAll('input[name="start_revision"], input[name="end_revision"]')
-    .forEach(function(radio) { radio.addEventListener('change', update_diff_button_state); });
+    // Two-slot FIFO: the newest pick becomes the target and the previous
+    // target becomes the source, so two taps anywhere give any pair.
+    // Picking the current target is a no-op; picking the current source
+    // swaps them.
+    var stack = [from_field.value, to_field.value].filter(function (v) { return v !== ''; });
 
-  update_diff_button_state();
+    function render() {
+      var from = stack[0] || '';
+      var to   = stack[1] || '';
+      from_field.value = from;
+      to_field.value   = to;
+      from_label.textContent = from;
+      to_label.textContent   = to;
+      readout.hidden  = !(from && to);
+      submit.disabled = !(from && to && from !== to);
 
-  document.getElementById('diff_form').addEventListener('submit', function(e) {
-    var start = document.querySelector('input[name="start_revision"]:checked');
-    var end = document.querySelector('input[name="end_revision"]:checked');
-    var view = document.querySelector('input[name="view"]:checked');
-    if (start) {
-      var s = document.createElement('input');
-      s.type = 'hidden'; s.name = 'start_revision'; s.value = start.value;
-      this.appendChild(s);
-    }
-    if (end) {
-      var en = document.createElement('input');
-      en.type = 'hidden'; en.name = 'end_revision'; en.value = end.value;
-      this.appendChild(en);
+      rows.forEach(function (row) {
+        var rev = row.dataset.revision;
+        row.classList.toggle('diff_source', rev === from);
+        row.classList.toggle('diff_target', rev === to);
+        row.setAttribute('aria-pressed', (rev === from || rev === to) ? 'true' : 'false');
+      });
     }
-    if (view) {
-      var v = document.createElement('input');
-      v.type = 'hidden'; v.name = 'view'; v.value = view.value;
-      this.appendChild(v);
+
+    function pick(rev) {
+      if (rev === stack[1]) { return; }
+      stack.push(rev);
+      if (stack.length > 2) { stack.shift(); }
+      render();
     }
-  });
+
+    rows.forEach(function (row) {
+      row.addEventListener('click', function (e) {
+        if (e.target.closest('a, button, input, label')) { return; }
+        pick(row.dataset.revision);
+      });
+      row.addEventListener('keydown', function (e) {
+        if (e.key === 'Enter' || e.key === ' ') {
+          e.preventDefault();
+          pick(row.dataset.revision);
+        }
+      });
+    });
+
+    render();
+  })();
 <% end %>
diff --git a/config/locales/de.yml b/config/locales/de.yml
index 3f2eb0ab..35f262cc 100644
--- a/config/locales/de.yml
+++ b/config/locales/de.yml
@@ -757,8 +757,7 @@ de:
       first: "Von"
       last: "Bis"
       diff_revisions: "Revisionen vergleichen"
-      against_word: "mit"
-      selected_word: "ausgewählt"
+      diff_selection_html: "Vergleicht %{from} mit %{to}"
       show_link: "anzeigen"
       restore_link: "wiederherstellen"
       confirm_restore: "Diese Revision wiederherstellen?"
diff --git a/config/locales/en.yml b/config/locales/en.yml
index e8428f69..75c762a3 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -725,8 +725,7 @@ en:
       first: "First"
       last: "Last"
       diff_revisions: "Diff revisions"
-      against_word: "against"
-      selected_word: "selected"
+      diff_selection_html: "Comparing %{to} against %{from}"
       show_link: "show"
       restore_link: "restore"
       confirm_restore: "Restore this revision?"
diff --git a/public/stylesheets/admin.css b/public/stylesheets/admin.css
index d3f3afe6..8bbf7289 100644
--- a/public/stylesheets/admin.css
+++ b/public/stylesheets/admin.css
@@ -686,6 +686,28 @@ form.button_to svg  {
   border-bottom: 1px solid var(--hairline);
 }
 
+.revisions_table tbody tr {
+  cursor: pointer;
+}
+
+.revisions_table tbody tr.diff_source {
+  background-color: var(--diff-del-bg);
+}
+
+.revisions_table tbody tr.diff_target {
+  background-color: var(--diff-ins-bg);
+}
+
+#diff_selection_label #diff_from {
+  color: var(--diff-del-fg);
+  font-weight: bold;
+}
+
+#diff_selection_label #diff_to {
+  color: var(--diff-ins-fg);
+  font-weight: bold;
+}
+
 .row_primary {
   font-weight: bold;
 }