Improve bandwidth estimation and adaptive switching with smaller segm…

…ents and higher TTFB

Fixes #3578 (special thanks to @Oleksandr0xB for submitting #4283)
Fixes #3563 and Closes #3595 (special thanks to @kanongil)

api-extractor/report/hls.js.api.md

@@ -219,6 +219,16 @@ export interface BufferFlushingData {
     type: SourceBufferName | null;
 }
 
+// Warning: (ae-missing-release-tag) "BufferInfo" is exported by the package, but it is missing a release tag (@alpha, @beta, @public, or @internal)
+//
+// @public (undocumented)
+export type BufferInfo = {
+    len: number;
+    start: number;
+    end: number;
+    nextStart?: number;
+};
+
 // Warning: (ae-missing-release-tag) "CapLevelControllerConfig" is exported by the package, but it is missing a release tag (@alpha, @beta, @public, or @internal)
 //
 // @public (undocumented)
@@ -922,6 +932,8 @@ class Hls implements HlsEventEmitter {
     get lowLatencyMode(): boolean;
     // Warning: (ae-setter-with-docs) The doc comment for the property "lowLatencyMode" must appear on the getter, not the setter.
     set lowLatencyMode(mode: boolean);
+    // (undocumented)
+    get mainForwardBufferInfo(): BufferInfo | null;
     get manualLevel(): number;
     get maxAutoLevel(): number;
     get maxLatency(): number;
@@ -965,6 +977,7 @@ class Hls implements HlsEventEmitter {
     get targetLatency(): number | null;
     // (undocumented)
     trigger<E extends keyof HlsListeners>(event: E, eventObject: Parameters<HlsListeners[E]>[1]): boolean;
+    get ttfbEstimate(): number;
     // (undocumented)
     readonly userConfig: Partial<HlsConfig>;
     // (undocumented)

demo/main.js

@@ -1212,6 +1212,7 @@ function checkBuffer() {
           log += `Dropped frames: ${video.webkitDroppedFrameCount}\n`;
         }
       }
+      log += `TTFB Estimate: ${hls.ttfbEstimate.toFixed(3)}\n`;
       log += `Bandwidth Estimate: ${hls.bandwidthEstimate.toFixed(3)}\n`;
       if (events.isLive) {
         log +=