1 files changed, 89 insertions, 0 deletions
diff --git a/frontend-old/node_modules/web-vitals/src/lib/getVisibilityWatcher.ts b/frontend-old/node_modules/web-vitals/src/lib/getVisibilityWatcher.ts
new file mode 100644
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+++ b/frontend-old/node_modules/web-vitals/src/lib/getVisibilityWatcher.ts
@@ -0,0 +1,89 @@
+/*
+ * Copyright 2020 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import {onBFCacheRestore} from './bfcache.js';
+
+let firstHiddenTime = -1;
+
+const initHiddenTime = () => {
+  // If the document is hidden when this code runs, assume it was always
+  // hidden and the page was loaded in the background, with the one exception
+  // that visibility state is always 'hidden' during prerendering, so we have
+  // to ignore that case until prerendering finishes (see: `prerenderingchange`
+  // event logic below).
+  return document.visibilityState === 'hidden' && !document.prerendering
+    ? 0
+    : Infinity;
+};
+
+const onVisibilityUpdate = (event: Event) => {
+  // If the document is 'hidden' and no previous hidden timestamp has been
+  // set, update it based on the current event data.
+  if (document.visibilityState === 'hidden' && firstHiddenTime > -1) {
+    // If the event is a 'visibilitychange' event, it means the page was
+    // visible prior to this change, so the event timestamp is the first
+    // hidden time.
+    // However, if the event is not a 'visibilitychange' event, then it must
+    // be a 'prerenderingchange' event, and the fact that the document is
+    // still 'hidden' from the above check means the tab was activated
+    // in a background state and so has always been hidden.
+    firstHiddenTime = event.type === 'visibilitychange' ? event.timeStamp : 0;
+
+    // Remove all listeners now that a `firstHiddenTime` value has been set.
+    removeChangeListeners();
+  }
+};
+
+const addChangeListeners = () => {
+  addEventListener('visibilitychange', onVisibilityUpdate, true);
+  // IMPORTANT: when a page is prerendering, its `visibilityState` is
+  // 'hidden', so in order to account for cases where this module checks for
+  // visibility during prerendering, an additional check after prerendering
+  // completes is also required.
+  addEventListener('prerenderingchange', onVisibilityUpdate, true);
+};
+
+const removeChangeListeners = () => {
+  removeEventListener('visibilitychange', onVisibilityUpdate, true);
+  removeEventListener('prerenderingchange', onVisibilityUpdate, true);
+};
+
+export const getVisibilityWatcher = () => {
+  if (firstHiddenTime < 0) {
+    // If the document is hidden when this code runs, assume it was hidden
+    // since navigation start. This isn't a perfect heuristic, but it's the
+    // best we can do until an API is available to support querying past
+    // visibilityState.
+    firstHiddenTime = initHiddenTime();
+    addChangeListeners();
+
+    // Reset the time on bfcache restores.
+    onBFCacheRestore(() => {
+      // Schedule a task in order to track the `visibilityState` once it's
+      // had an opportunity to change to visible in all browsers.
+      // https://bugs.chromium.org/p/chromium/issues/detail?id=1133363
+      setTimeout(() => {
+        firstHiddenTime = initHiddenTime();
+        addChangeListeners();
+      }, 0);
+    });
+  }
+  return {
+    get firstHiddenTime() {
+      return firstHiddenTime;
+    },
+  };
+};