INP attribution: ensure LoAF order is maintained (#512)

Author: brendankenny

src/attribution/onINP.ts

@@ -63,7 +63,7 @@ let pendingLoAFs: PerformanceLongAnimationFrameTiming[] = [];
 let pendingEntriesGroups: pendingEntriesGroup[] = [];
 
 // The `processingEnd` time of most recently-processed event, chronologically.
-let latestProcessingEnd: number;
+let latestProcessingEnd: number = 0;
 
 // A WeakMap to look up the event-timing-entries group of a given entry.
 // Note that this only maps from "important" entries: either the first input or
@@ -188,7 +188,7 @@ const cleanupEntries = () => {
 
   // Keep all pending LoAF entries that either:
   // 1) intersect with entries in the newly cleaned up `pendingEntriesGroups`
-  // 2) occur after the most recently-processed event entry.
+  // 2) occur after the most recently-processed event entry (for up to MAX_PREVIOUS_FRAMES)
   const loafsToKeep: Set<PerformanceLongAnimationFrameTiming> = new Set();
   for (let i = 0; i < pendingEntriesGroups.length; i++) {
     const group = pendingEntriesGroups[i];
@@ -198,18 +198,15 @@ const cleanupEntries = () => {
       },
     );
   }
-  for (let i = 0; i < MAX_PREVIOUS_FRAMES; i++) {
-    // Look at pending LoAF in reverse order so the most recent are first.
-    const loaf = pendingLoAFs[pendingLoAFs.length - 1 - i];
-
-    // If we reach LoAFs that overlap with event processing,
-    // we can assume all previous ones have already been handled.
-    if (!loaf || loaf.startTime < latestProcessingEnd) break;
-
-    loafsToKeep.add(loaf);
-  }
+  const prevFrameIndexCutoff = pendingLoAFs.length - 1 - MAX_PREVIOUS_FRAMES;
+  // Filter `pendingLoAFs` to preserve LoAF order.
+  pendingLoAFs = pendingLoAFs.filter((loaf, index) => {
+    if (loaf.startTime > latestProcessingEnd && index > prevFrameIndexCutoff) {
+      return true;
+    }
 
-  pendingLoAFs = Array.from(loafsToKeep);
+    return loafsToKeep.has(loaf);
+  });
 
   // Reset the idle callback handle so it can be queued again.
   idleHandle = -1;