Prevent LCP being reported for hidden prerendered page (#326)

src/onLCP.ts

@@ -54,17 +54,18 @@ export const onLCP = (onReport: ReportCallback, opts?: ReportOpts) => {
     const handleEntries = (entries: LCPMetric['entries']) => {
       const lastEntry = entries[entries.length - 1] as LargestContentfulPaint;
       if (lastEntry) {
-        // The startTime attribute returns the value of the renderTime if it is
-        // not 0, and the value of the loadTime otherwise. The activationStart
-        // reference is used because LCP should be relative to page activation
-        // rather than navigation start if the page was prerendered. But in cases
-        // where `activationStart` occurs after the LCP, this time should be
-        // clamped at 0.
-        const value = Math.max(lastEntry.startTime - getActivationStart(), 0);
-
         // Only report if the page wasn't hidden prior to LCP.
-        if (value < visibilityWatcher.firstHiddenTime) {
-          metric.value = value;
+        if (lastEntry.startTime < visibilityWatcher.firstHiddenTime) {
+          // The startTime attribute returns the value of the renderTime if it is
+          // not 0, and the value of the loadTime otherwise. The activationStart
+          // reference is used because LCP should be relative to page activation
+          // rather than navigation start if the page was prerendered. But in cases
+          // where `activationStart` occurs after the LCP, this time should be
+          // clamped at 0.
+          metric.value = Math.max(
+            lastEntry.startTime - getActivationStart(),
+            0
+          );
           metric.entries = [lastEntry];
           report();
         }