Add visibility-aware interval helper

frontend/visible-timer.js

@@ -0,0 +1,41 @@
+(function (root) {
+  'use strict';
+
+  function setIntervalWhenVisible(fn, ms, { immediate = false } = {}) {
+    if (typeof fn !== 'function') throw new TypeError('fn must be a function');
+    const doc = root.document;
+    if (!doc) throw new Error('document is required');
+
+    let timer = null;
+    let stopped = false;
+    const clear = () => {
+      if (timer !== null) root.clearInterval(timer);
+      timer = null;
+    };
+    const start = (runNow) => {
+      if (stopped || doc.hidden) return;
+      clear();
+      if (runNow) fn();
+      timer = root.setInterval(fn, ms);
+    };
+    const onVisibilityChange = () => {
+      if (doc.hidden) clear();
+      else start(immediate);
+    };
+
+    if (!doc.hidden) start(immediate);
+    doc.addEventListener('visibilitychange', onVisibilityChange);
+    return {
+      clear() {
+        if (stopped) return;
+        stopped = true;
+        clear();
+        doc.removeEventListener('visibilitychange', onVisibilityChange);
+      },
+    };
+  }
+
+  const api = { setIntervalWhenVisible };
+  if (typeof module !== 'undefined' && module.exports) module.exports = api;
+  else root.VisibleTimer = api;
+})(globalThis);