From a143c8331dfa396956f2d8dfdd388bbb42961ab6 Mon Sep 17 00:00:00 2001
From: Jonathan Sykes <telemagnadon@gmail.com>
Date: Tue, 6 Oct 2026 09:14:06 +0800
Subject: [PATCH] Stop video seek loop when returning from background

---
 frontend/app.js | 34 +++++++++++++++++++++++++++++-----
 1 file changed, 29 insertions(+), 5 deletions(-)

diff --git a/frontend/app.js b/frontend/app.js
index fa9ff1f..a213db2 100755
--- a/frontend/app.js
+++ b/frontend/app.js
@@ -2704,6 +2704,24 @@ const Player = {
   SOFT_SYNC_THRESHOLD: 0.08,
   HARD_SYNC_THRESHOLD: 0.3,
   SLEW_RATE: 0.04, // playbackRate offset applied while easing back into sync
+  SNAP_COOLDOWN_MS: 3000, // after a hard snap, only slew for this long
+  _seekLag: 0.25,         // seconds the last video seek took (measured), used as the snap lead
+
+  // Seek the muted video to where the sounding clock WILL be once the seek lands,
+  // measuring how long seeks take on this device so the lead adapts.
+  snapVideoTo(V, clock) {
+    const rate = (clock && clock.playbackRate) || 1;
+    const lead = clock && !clock.paused ? Math.min(1.5, this._seekLag) * rate : 0;
+    const target = ((clock && clock.currentTime) || 0) + lead;
+    const started = performance.now();
+    this._internalSeek = true;
+    this._lastSnapAt = Date.now();
+    V.addEventListener('seeked', () => {
+      const took = (performance.now() - started) / 1000;
+      this._seekLag = Math.max(0.05, Math.min(1.5, this._seekLag * 0.5 + took * 0.5));
+    }, { once: true });
+    V.currentTime = target;
+  },
 
   // Shared by the 1s watchdog tick and the master 'playing' handler so both
   // paths get the same no-pop behavior.
@@ -2719,12 +2737,18 @@ const Player = {
   correctDrift() {
     if (this.mode !== 'dual' || !this.secondary || this.master.paused) return;
     if (this.secondary.paused) return; // resume paths own this case
+    // A seek still in flight is not drift: re-snapping now restarts the seek and,
+    // on iPhone, loops "seek → land behind → seek again" (stutter after resuming).
+    if (this.master.seeking) return;
     const drift = this.secondary.currentTime - this.master.currentTime;
     const abs = Math.abs(drift);
-    if (abs >= this.HARD_SYNC_THRESHOLD) {
+    if (abs >= this.HARD_SYNC_THRESHOLD && Date.now() - (this._lastSnapAt || 0) < this.SNAP_COOLDOWN_MS) {
+      // Just snapped: ease in instead of snapping again.
+      const base = parseFloat(els.speed.value) || 1;
+      this.master.playbackRate = drift > 0 ? base + this.SLEW_RATE * 2 : base - this.SLEW_RATE * 2;
+    } else if (abs >= this.HARD_SYNC_THRESHOLD) {
       // Snap the muted video onto the audio clock (inaudible).
-      this._internalSeek = true;
-      this.master.currentTime = this.secondary.currentTime;
+      this.snapVideoTo(this.master, this.secondary);
       this._resyncSpeed();
     } else if (abs >= this.SOFT_SYNC_THRESHOLD) {
       // Video behind audio -> speed the video up; ahead -> slow it down.
@@ -2841,7 +2865,7 @@ const Player = {
       this.master = V;
       this.secondary = A;
       try {
-        if (AudioContinuity.needsAlignment(V.currentTime, t, 0.12)) { this._internalSeek = true; V.currentTime = t; }
+        if (AudioContinuity.needsAlignment(V.currentTime, t, 0.12)) this.snapVideoTo(V, A);
         if (this._wantsPlaying) audioContinuity.play(V);
       } catch { /* the watchdog picks up whatever is left */ }
       return;
@@ -2853,7 +2877,7 @@ const Player = {
       this.mode = 'dual';
       this.secondary = A;
       V.muted = true;
-      if (AudioContinuity.needsAlignment(V.currentTime, t, 0.12)) { this._internalSeek = true; V.currentTime = t; }
+      if (AudioContinuity.needsAlignment(V.currentTime, t, 0.12)) this.snapVideoTo(V, A);
       if (this._wantsPlaying) audioContinuity.play(V);
     } catch { /* the watchdog picks up whatever is left */ }
   },