Fix Sundry Creditors follow-up FY dropdown

app/modules/accounting/templates/accounting/sundry_creditors_aging.html

@@ -21,12 +21,12 @@
       </select>
     </label>
     <label class="space-y-1 text-sm"><span class="font-semibold text-slate-700">Analysis FY</span>
-      <select name="financial_year" class="w-full rounded-xl border border-slate-300 px-3 py-2">
+      <select name="financial_year" data-creditor-analysis-fy class="w-full rounded-xl border border-slate-300 px-3 py-2">
         {% for fy in fy_options %}<option value="{{ fy }}" {% if fy == selected_fy %}selected{% endif %}>{{ fy }}</option>{% endfor %}
       </select>
     </label>
     <label class="space-y-1 text-sm"><span class="font-semibold text-slate-700">Follow-up FY (optional)</span>
-      <select name="follow_up_fy" class="w-full rounded-xl border border-slate-300 px-3 py-2">
+      <select name="follow_up_fy" data-creditor-follow-up-fy class="w-full rounded-xl border border-slate-300 px-3 py-2">
         <option value="">Do not trace later payments</option>
         {% for fy in fy_options %}{% if fy > selected_fy %}<option value="{{ fy }}" {% if fy == follow_up_fy %}selected{% endif %}>{{ fy }}</option>{% endif %}{% endfor %}
       </select>
@@ -143,6 +143,67 @@
 
 <script>
 (function () {
+  // Keep Follow-up FY synchronized with the Analysis FY on the client side.
+  // Previously the server rendered follow-up options only for the FY that was
+  // active when the page loaded. If the user changed Analysis FY in the form
+  // (for example 2026-27 -> 2021-22), the Follow-up FY control still showed
+  // 2027-28 until the page was submitted. Rebuild it immediately from the FY
+  // values already supplied by the server, without changing any accounting or
+  // FIFO calculation logic.
+  var analysisFySelect = document.querySelector('[data-creditor-analysis-fy]');
+  var followUpFySelect = document.querySelector('[data-creditor-follow-up-fy]');
+
+  function fyStartYear(value) {
+    var match = String(value || '').match(/^(\d{4})-\d{2}$/);
+    return match ? parseInt(match[1], 10) : null;
+  }
+
+  if (analysisFySelect && followUpFySelect) {
+    var fyUniverse = Array.from(analysisFySelect.options)
+      .map(function (option) { return String(option.value || '').trim(); })
+      .filter(function (value) { return fyStartYear(value) !== null; });
+
+    function rebuildFollowUpYears() {
+      var selectedFy = String(analysisFySelect.value || '').trim();
+      var selectedStart = fyStartYear(selectedFy);
+      var previousFollowUp = String(followUpFySelect.value || '').trim();
+
+      while (followUpFySelect.options.length) {
+        followUpFySelect.remove(0);
+      }
+
+      var emptyOption = document.createElement('option');
+      emptyOption.value = '';
+      emptyOption.textContent = 'Do not trace later payments';
+      followUpFySelect.appendChild(emptyOption);
+
+      if (selectedStart === null) return;
+
+      var laterYears = fyUniverse
+        .filter(function (fy) {
+          var start = fyStartYear(fy);
+          return start !== null && start > selectedStart;
+        })
+        .sort(function (a, b) { return fyStartYear(a) - fyStartYear(b); });
+
+      laterYears.forEach(function (fy) {
+        var option = document.createElement('option');
+        option.value = fy;
+        option.textContent = fy;
+        followUpFySelect.appendChild(option);
+      });
+
+      if (laterYears.indexOf(previousFollowUp) !== -1) {
+        followUpFySelect.value = previousFollowUp;
+      } else {
+        followUpFySelect.value = '';
+      }
+    }
+
+    analysisFySelect.addEventListener('change', rebuildFollowUpYears);
+    rebuildFollowUpYears();
+  }
+
   function applyBucket(detailRow, bucket) {
     if (!detailRow) return;
     detailRow.querySelectorAll('[data-creditor-lot]').forEach(function (lot) {