perf: voice list sessionStorage cache — instant render on reload (v1.12.20)

After a successful /api/voices fetch, results are written to sessionStorage
(key ttsvc_vc). On the next page load loadVoiceLibrary() reads the cache and
renders voices immediately before the network request completes, eliminating
the blank-list / skeleton flash entirely.

The background fetch always runs and overwrites the cache with fresh data.
Error handling is graceful: if the network fails but cache was served, the
error is suppressed (stale data stays visible). The Refresh button clears the
cache first to force a full reload cycle.

Silent-refresh logic (from v1.12.19) prevents the second loadVoiceLibrary()
call (engines.js batch E) from blanking already-rendered voices.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -9,6 +9,13 @@ Follows [Keep a Changelog](https://keepachangelog.com/en/1.0.0/) · versioned wi
 
 ---
 
+## [1.12.20] — 2026-06-28
+
+### Performance
+- **Voice list sessionStorage cache** — voices are written to `sessionStorage` after every successful fetch (`ttsvc_vc`). On the next page load the list renders instantly from cache while the fresh fetch runs silently in the background. The Refresh button clears the cache first to guarantee a clean reload.
+
+---
+
 ## [1.12.19] — 2026-06-28
 
 ### Fixed

VERSION

@@ -1 +1 @@
-1.12.19
+1.12.20

static/index.html

@@ -26,7 +26,7 @@
 
   <!-- ── Core styles (local — no CDN dependency for first paint) ────────── -->
   <link rel="stylesheet" href="/static/vendor/mdi/materialdesignicons.min.css">
-  <link rel="stylesheet" href="/static/style.css?v=1.12.19">
+  <link rel="stylesheet" href="/static/style.css?v=1.12.20">
 
 
   <!-- ── Flag icons — non-blocking (loaded async, icons appear after JS) ── -->
@@ -336,7 +336,7 @@
 <script src="/static/vendor/wavesurfer-regions.min.js"></script>
 
 <!-- loader.js: fetches sections → loads JS modules → removes skeleton -->
-<script src="/static/loader.js?v=1.12.19"></script>
+<script src="/static/loader.js?v=1.12.20"></script>
 
 </body>
 </html>

static/js/voice-library.js

@@ -42,6 +42,18 @@ let _activePlayVoiceId = null;
 let _activePlayUrl = null;
 let _libraryLoadPromise = null;
 const BENCHMARK_SAMPLE_STORAGE_KEY = 'vcf-benchmark-sample-text';
+const _VL_CACHE_KEY = 'ttsvc_vc';
+
+function _vlCacheRead() {
+  try { return JSON.parse(sessionStorage.getItem(_VL_CACHE_KEY) || 'null'); } catch (_) { return null; }
+}
+function _vlCacheWrite(voices) {
+  try { sessionStorage.setItem(_VL_CACHE_KEY, JSON.stringify(voices)); } catch (_) {}
+}
+function _vlCacheClear() {
+  try { sessionStorage.removeItem(_VL_CACHE_KEY); } catch (_) {}
+}
+window._vlCacheClear = _vlCacheClear;
 const _libraryFilters = {text:'', lang:'', sex:'', type:'', rating:''};
 const DEFAULT_BENCHMARK_SAMPLE_TEXT = 'Hello, how are you today? Please read this sample clearly for a fair voice benchmark.';
 
@@ -613,25 +625,42 @@ async function loadVoiceLibrary() {
   _libraryLoadPromise = (async () => {
     setBusyButton('refresh-voices-btn', true);
     const list = $('voice-list');
-    const silent = _voices.length > 0; // already rendered — refresh without blanking the list
+
+    // Serve from sessionStorage cache immediately so the list is never blank on reload
+    const cached = _voices.length === 0 ? _vlCacheRead() : null;
+    if (cached && Array.isArray(cached) && cached.length) {
+      _voices = cached;
+      window._voices = _voices;
+      if (typeof window.updateVoiceTree === 'function') window.updateVoiceTree(_voices);
+      renderVoiceList();
+      updatePreviewVoiceMatchPanel();
+      status(`Loaded ${_voices.length} voices`);
+    }
+
+    // Always background-fetch fresh data; skip skeleton if something is already shown
+    const silent = _voices.length > 0;
     if (list && !silent) list.innerHTML = loadingMarkup('Loading voice library', 'Scanning voices, reference text, metadata, ratings, and benchmark results.', 8);
     if (!silent) { $('voice-count').textContent = 'Loading voices…'; updateLibraryInsights('loading'); status('Loading voice library…'); }
     try {
       const r = await fetch('/api/voices');
       if (!r.ok) { const e = await r.json().catch(() => ({})); throw new Error(e.detail || r.statusText); }
-      _voices = await r.json();
+      const fresh = await r.json();
-      window._voices = _voices; // expose for cross-module access (Script Rehearser etc.)
+      _vlCacheWrite(fresh);
+      _voices = fresh;
+      window._voices = _voices;
       if (typeof window.updateVoiceTree === 'function') window.updateVoiceTree(_voices);
       renderVoiceList();
       updatePreviewVoiceMatchPanel();
       if (typeof renderPerfHistory === 'function') renderPerfHistory();
       status(`Loaded ${_voices.length} voices`);
     } catch(e) {
+      if (!silent) {
         if (list) list.innerHTML = '<div style="color:var(--red);padding:8px;font-size:13px">Failed to load voices: ' + (e.message||String(e)) + '</div>';
         $('voice-count').textContent = 'Load failed';
         updateLibraryInsights('error');
+      }
       status('Voice library load failed: ' + e.message);
-      throw e;
+      if (!silent) throw e;
     } finally {
       setBusyButton('refresh-voices-btn', false);
       _libraryLoadPromise = null;
@@ -640,7 +669,7 @@ async function loadVoiceLibrary() {
   return _libraryLoadPromise;
 }
 
-$('refresh-voices-btn').addEventListener('click', loadVoiceLibrary);
+$('refresh-voices-btn').addEventListener('click', () => { _vlCacheClear(); loadVoiceLibrary(); });
 $('sync-voice-folders-btn').addEventListener('click', async () => {
   $('sync-voice-folders-btn').disabled = true;
   status('Syncing active_voices and hidden_voices…');