Don't append over first fragment when next fragment aligns with playl…

…ist within 1/200s tolerance

Fixes edge-case starting in v1.5 that causes #6441

src/controller/base-stream-controller.ts

@@ -1278,7 +1278,7 @@ export default class BaseStreamController
     let { fragPrevious } = this;
     let { fragments, endSN } = levelDetails;
     const { fragmentHint } = levelDetails;
-    const tolerance = config.maxFragLookUpTolerance;
+    const { maxFragLookUpTolerance } = config;
     const partList = levelDetails.partList;
 
     const loadingParts = !!(
@@ -1294,7 +1294,8 @@ export default class BaseStreamController
 
     let frag;
     if (bufferEnd < end) {
-      const lookupTolerance = bufferEnd > end - tolerance ? 0 : tolerance;
+      const lookupTolerance =
+        bufferEnd > end - maxFragLookUpTolerance ? 0 : maxFragLookUpTolerance;
       // Remove the tolerance if it would put the bufferEnd past the actual end of stream
       // Uses buffer and sequence number to calculate switch segment (required if using EXT-X-DISCONTINUITY-SEQUENCE)
       frag = findFragmentByPTS(

src/controller/fragment-finders.ts

@@ -58,6 +58,7 @@ export function findFragmentByPTS(
   fragments: Array<Fragment>,
   bufferEnd: number = 0,
   maxFragLookUpTolerance: number = 0,
+  nextFragLookupTolerance: number = 0.005,
 ): Fragment | null {
   let fragNext: Fragment | null = null;
   if (fragPrevious) {
@@ -76,9 +77,17 @@ export function findFragmentByPTS(
   // Prefer the next fragment if it's within tolerance
   if (
     fragNext &&
-    (!fragPrevious || fragPrevious.level === fragNext.level) &&
-    fragmentWithinToleranceTest(bufferEnd, maxFragLookUpTolerance, fragNext) ===
-      0
+    (((!fragPrevious || fragPrevious.level === fragNext.level) &&
+      fragmentWithinToleranceTest(
+        bufferEnd,
+        maxFragLookUpTolerance,
+        fragNext,
+      ) === 0) ||
+      fragmentWithinFastStartSwitch(
+        fragNext,
+        fragPrevious,
+        Math.min(nextFragLookupTolerance, maxFragLookUpTolerance),
+      ))
   ) {
     return fragNext;
   }
@@ -94,6 +103,28 @@ export function findFragmentByPTS(
   return fragNext;
 }
 
+function fragmentWithinFastStartSwitch(
+  fragNext: Fragment,
+  fragPrevious: Fragment | null,
+  nextFragLookupTolerance: number,
+): boolean {
+  if (
+    fragPrevious &&
+    fragPrevious.start === 0 &&
+    fragPrevious.level < fragNext.level &&
+    (fragPrevious.endPTS || 0) > 0
+  ) {
+    const firstDuration = fragPrevious.tagList.reduce((duration, tag) => {
+      if (tag[0] === 'INF') {
+        duration += parseFloat(tag[1]);
+      }
+      return duration;
+    }, nextFragLookupTolerance);
+    return fragNext.start <= firstDuration;
+  }
+  return false;
+}
+
 /**
  * The test function used by the findFragmentBySn's BinarySearch to look for the best match to the current buffer conditions.
  * @param candidate - The fragment to test