fix(sampler_core): gate takeover declick by (1-amp) to remove zero-attack +6 dB blip and long-attack notch; clamp seed to +/-1; add zero-attack test

src/vst/sampler_core.cpp

@@ -272,8 +272,10 @@ void Voice::start(int note, int velocity, const SampleData& sample, int rootNote
     // the same pre-cut output level — zeroing would drop the pending ramp and bring the
     // click back on that edge. The next rendered frame overwrites lastOut anyway.
     if (declickTakeover && active_) {
-        declickL_ = lastOutL_;
-        declickR_ = lastOutR_;
+        // Clamp the seed to ±1.0: closes the theoretical same-frame-repeat accumulation edge
+        // (successive starts before any frame is rendered cannot grow the seed above full scale).
+        declickL_ = (lastOutL_ > 1.0) ? 1.0 : (lastOutL_ < -1.0) ? -1.0 : lastOutL_;
+        declickR_ = (lastOutR_ > 1.0) ? 1.0 : (lastOutR_ < -1.0) ? -1.0 : lastOutR_;
         declickActive_ = (declickL_ > kDeclickFloor || declickL_ < -kDeclickFloor ||
                           declickR_ > kDeclickFloor || declickR_ < -kDeclickFloor);
     } else {
@@ -530,9 +532,16 @@ AudioSample Voice::advanceFrame(bool stereo, AudioSample& outR) {
     // takeover/steal start() so the restart's hard cut has no step. Engine-agnostic — applied
     // after either pitch-engine branch, on the shared epilogue. Inactive (the common case)
     // costs one branch.
+    //
+    // Gate by (1 - amp): the compensation fills only the HOLE the new attack leaves. When amp
+    // is near 0 (slow attack, typical 3 ms) the gate is ~1 — full compensation, no change in
+    // feel. When amp is 1 (zero-attack, instant sustain) the gate is 0 — no compensation added,
+    // so there is no +6 dB blip. For long attacks the gate tapers the compensation proportionally,
+    // removing the notch that arose when both the old tail and the new level were present in full.
     if (declickActive_) {
-        outL += declickL_;
-        if (stereo) outRlocal += declickR_;
+        const double gate = 1.0 - amp;
+        outL += declickL_ * gate;
+        if (stereo) outRlocal += declickR_ * gate;
         declickL_ *= kDeclickDecay;
         declickR_ *= kDeclickDecay;
         if (declickL_ < kDeclickFloor && declickL_ > -kDeclickFloor &&

tests/test_sampler_core.cpp

@@ -2063,6 +2063,35 @@ static void testSameBlockDoubleTakeoverKeepsDeclickSeed() {
     CHECK(approx(post.back(), 1.0, 1e-3));
 }
 
+// Declick with ZERO-ATTACK takeover: the new voice reaches full level on frame 0 (amp == 1),
+// so the gated compensation adds nothing (gate = 1 - 1 = 0). Output on the boundary frame is
+// exactly the new voice's level, never exceeding full scale. Without the (1-amp) gate a zero-
+// attack takeover from a sustained voice produced newOnset + oldLevel -> up to 2x (+6 dB).
+static void testZeroAttackTakeoverNeverExceedsFullScale() {
+    SampleData s = dcSample(200000, 60);
+    s.play.adsr.attackFrames = 0;     // zero-attack: amp == 1 on the very first frame
+    s.play.adsr.sustainLevel = 1.0;
+    s.play.adsr.releaseFrames = 0;
+    Keymap km = Keymap::singleSampleChromatic(std::move(s));
+    VoiceEngine eng(1, km, 0, 0, VoiceMode::Mono, MonoTrigger::Retrigger,
+                    /*takeoverDeclick=*/true);
+
+    eng.noteOn(60, 127);
+    std::vector<AudioSample> pre;
+    eng.render(pre, 200);             // sustained at 1.0
+    CHECK(approx(pre.back(), 1.0, 1e-6));
+
+    eng.noteOn(64, 127);              // zero-attack takeover: amp hits 1 on frame 0
+    std::vector<AudioSample> post;
+    eng.render(post, 400);
+    // Every output frame must stay within [-1, 1]: no +6 dB blip.
+    for (AudioSample v : post) {
+        CHECK(v <= 1.0f + 1e-4f && v >= -1.0f - 1e-4f);
+    }
+    // The zero-attack note settles at sustain 1.0 immediately.
+    CHECK(approx(post[0], 1.0, 1e-4));
+}
+
 // GA-VoiceSteal repro (DAW bug): voiceCount 3, a triad note-on'd at the SAME sample time
 // (three note-ons in one block, no render between), then a 4th note. The steal must take
 // EXACTLY ONE voice (the oldest, none releasing) and leave the other two RINGING — the DAW
@@ -2265,6 +2294,7 @@ int main() {
     testMonoDeclickOnlyOnTakeover();
     testPolyStealDeclicksRestart();
     testSameBlockDoubleTakeoverKeepsDeclickSeed();
+    testZeroAttackTakeoverNeverExceedsFullScale();
     testPreviewCardIsolatedFromPool();
     testPreviewCardReplaceStaleOffAndOutOfZone();