diff --git a/static/js/app.js b/static/js/app.js
index f0a0c31..5068fff 100644
--- a/static/js/app.js
+++ b/static/js/app.js
@@ -2731,6 +2731,10 @@ async function _setCachedBook(bookId, data_ct, data_iv) {
       tx.oncomplete = resolve;
       tx.onerror    = resolve;
     });
+    // Request persistent storage so the browser doesn't evict cached books
+    // under storage pressure. Chrome grants this silently for installed PWAs;
+    // Firefox may show a permission prompt.
+    if (navigator.storage?.persist) navigator.storage.persist();
   } catch {}
 }