loop: crossfade the Gate sustain seam, and unshadow the loop handles that made loop points look gone

src/shell/instrument/editor_input_waveform.cpp

@@ -52,20 +52,33 @@ bool ReaSamplerEditor::mouseDownWaveform(const FaceLayout& fl, int x, int y) {
         }
     }
     const SetupMarkers m = pickedMarkers(frames);
+    // The crossfade handle first, and only when there IS a loop to fade: at a zero fade it
+    // sits exactly on the loop start, so it can only stay reachable by owning the top strip
+    // (waveform_view.h's handle-vs-column split) and being asked first.
+    if (m.hasLoop &&
+        contains(markerHandleRect(overlay, frames, m.loopStart - m.crossfade), x, y)) {
+        beginMarkerDrag(WaveMarker::kLoopXfade, m, frames, x);
+        return true;
+    }
     const std::int64_t markerFrames[3] = {m.start, m.loopStart, m.loopEnd};
     const int hit = markerAtPoint(overlay, frames, markerFrames, 3, x, y);
     if (hit >= 0) {
-        drag_ = DragKind::kWaveMarker;
-        waveMarker_ = static_cast<WaveMarker>(hit);
-        dragStartX_ = x;
-        dragStartMarkers_ = m;
-        dragSampleFrames_ = frames;
-        dragStartParams_ = params_;
+        beginMarkerDrag(static_cast<WaveMarker>(hit), m, frames, x);
         return true;
     }
     return false;
 }
 
+void ReaSamplerEditor::beginMarkerDrag(WaveMarker which, const SetupMarkers& m,
+                                       std::int64_t frames, int x) {
+    drag_ = DragKind::kWaveMarker;
+    waveMarker_ = which;
+    dragStartX_ = x;
+    dragStartMarkers_ = m;
+    dragSampleFrames_ = frames;
+    dragStartParams_ = params_;
+}
+
 void ReaSamplerEditor::dragWaveform(const FaceLayout& fl, int x, int y) {
     const OverlayArea overlay = waveformOverlayArea(fl.bands.waveform);
     const int dx = x - dragStartX_;
@@ -96,14 +109,17 @@ void ReaSamplerEditor::dragWaveform(const FaceLayout& fl, int x, int y) {
 
     // Grabbed frame at grab time, from the snapshot (so the delta is measured from grab).
     const int idx = static_cast<int>(waveMarker_);
-    const std::int64_t startVals[3] = {dragStartMarkers_.start, dragStartMarkers_.loopStart,
-                                       dragStartMarkers_.loopEnd};
+    const std::int64_t startVals[4] = {dragStartMarkers_.start, dragStartMarkers_.loopStart,
+                                       dragStartMarkers_.loopEnd,
+                                       dragStartMarkers_.loopStart -
+                                           dragStartMarkers_.crossfade};
     std::int64_t newFrame = resolveDragFrame(overlay, frames, startVals[idx], dx);
 
     // Snap to the nearest zero crossing in the decoded PCM. Pure over the cached mono
-    // frames — no host types, no file I/O.
+    // frames — no host types, no file I/O. The crossfade handle is exempt: it sets a fade
+    // LENGTH, and the whole point of the fade is that its edges need no zero crossing.
     const std::vector<AudioSample>& pcm = monoPcmFor(selectedId_);
-    if (!pcm.empty()) {
+    if (!pcm.empty() && waveMarker_ != WaveMarker::kLoopXfade) {
         newFrame = nearestZeroCrossing(pcm.data(), static_cast<std::int64_t>(pcm.size()),
                                        newFrame);
     }
@@ -116,12 +132,19 @@ void ReaSamplerEditor::dragWaveform(const FaceLayout& fl, int x, int y) {
     } else if (waveMarker_ == WaveMarker::kLoopStart) {
         m.loopStart = (std::min)(newFrame, m.loopEnd);
         m.hasLoop = true;
-    } else {  // kLoopEnd
+    } else if (waveMarker_ == WaveMarker::kLoopEnd) {
         m.loopEnd = (std::max)(newFrame, m.loopStart);
         m.hasLoop = true;
+    } else {  // kLoopXfade — the handle sits at loopStart - crossfade, so left lengthens it
+        m.crossfade = (std::max)(std::int64_t{0}, m.loopStart - newFrame);
     }
     if (m.start < 0) m.start = 0;
     if (m.start > frames - 1) m.start = frames - 1;
+    // Mirror resolveLoop's own bound so the handle can't be dragged somewhere the engine
+    // would silently clamp back: the fade reads the material ahead of the loop, and cannot
+    // outrun either that material or the loop itself.
+    m.crossfade = (std::min)(m.crossfade, (std::min)(m.loopStart, m.loopEnd - m.loopStart));
+    if (m.crossfade < 0) m.crossfade = 0;
 
     applyMarkers(m);
     invalidate();  // live feedback; the commit lands on release