fix(live): keep HLS playback near the live edge

Normal 6-12s HLS kept the hls.js defaults (3 fragments back, no max
latency), so playback drifted far behind the edge. Target two
fragments, cap at four, and add a latency probe that snaps back when
past 20s.

src/components/LivePlayer.vue

@@ -46,6 +46,11 @@ import { computed, onBeforeUnmount, ref, watch } from 'vue'
 
 const STARTUP_RETRY_DELAYS_MS = [500, 1000, 2000, 4000, 6000, 8000]
 const STARTUP_WINDOW_MS = 30_000
+// 落后边缘的分片数上限；必须 > LIVE_SYNC_DURATION_COUNT，否则 hls.js 直接抛错
+const LIVE_SYNC_DURATION_COUNT = 2
+const LIVE_MAX_LATENCY_DURATION_COUNT = 4
+// 落后超过该秒数时强制跳到直播边缘（hls.js 内置 synchronizeToLiveEdge 的上限）
+const LIVE_EDGE_HARD_LIMIT_SEC = 20
 
 const props = defineProps({
   playUrl: { type: String, default: '' },
@@ -199,6 +204,22 @@ function scheduleSameUrlRetry(url, seq, httpCode = 404) {
   return true
 }
 
+function logLiveLatency() {
+  if (!hls) return
+  const latency = Number(hls.latency)
+  if (!Number.isFinite(latency) || latency <= 0) return
+  if (latency > LIVE_EDGE_HARD_LIMIT_SEC) {
+    console.warn(`[live-player] 落后直播边缘 ${latency.toFixed(1)}s，强制贴边`)
+    const video = videoEl.value
+    const syncPosition = hls.liveSyncPosition
+    if (video && Number.isFinite(syncPosition)) {
+      video.currentTime = syncPosition
+    }
+    return
+  }
+  console.debug(`[live-player] 落后直播边缘 ${latency.toFixed(1)}s`)
+}
+
 function bindMediaListeners(video, seq) {
   clearMediaListeners()
   const onPlay = () => {
@@ -328,10 +349,18 @@ async function attachPlayer(url, { seq: existingSeq } = {}) {
       return
     }
 
-    // 现网为普通 6–12s HLS，不是 LL-HLS；放宽分片重试，启动 404 仍由同 URL 窗口控制
+    // 现网为普通 6–12s HLS（非 LL-HLS）：lowLatencyMode 无 partHoldBack 可用，
+    // 靠 liveSyncDurationCount/liveMaxLatencyDurationCount 追边，重建后也能贴近边缘。
     hls = new Hls({
       enableWorker: true,
+      // 非 LL-HLS 下 true 无收益（partHoldBack=0 时回落 holdBack），保持关闭
       lowLatencyMode: false,
+      liveSyncDurationCount: LIVE_SYNC_DURATION_COUNT,
+      liveMaxLatencyDurationCount: LIVE_MAX_LATENCY_DURATION_COUNT,
+      maxLiveSyncPlaybackRate: 1.5,
+      maxBufferLength: 8,
+      // 0 会被内部抬到 1×targetduration，且卡顿后无法回退；留 30s 回退窗
+      backBufferLength: 30,
       manifestLoadingMaxRetry: 2,
       manifestLoadingRetryDelay: 1000,
       levelLoadingMaxRetry: 2,
@@ -409,6 +438,11 @@ async function attachPlayer(url, { seq: existingSeq } = {}) {
       video.muted = true
       tryPlay()
     })
+    // 观测落后直播边缘的秒数；重建后按需强制贴边
+    hls.on(Hls.Events.LEVEL_UPDATED, () => {
+      if (seq !== attachSeq) return
+      logLiveLatency()
+    })
   } catch (_) {
     if (seq === attachSeq) emitFatal(buildPlayError({ message: 'hls init failed' }))
   }