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

tests/test_waveform_view.cpp

@@ -5,6 +5,7 @@
 //
 // Covers: frameToX / xToFrame (linear map + inverse, edge clamps, degenerate frameCount/width);
 // markerAtPoint (grab band, first-match on overlap, off-area + null-array rejection);
+// markerHandleRect (the top-strip tab that keeps coincident markers independently grabbable);
 // resolveDragFrame (round-to-nearest-frame, clamp to [0,frameCount], zero-delta/zero-width
 // no-ops); nearestZeroCrossing (nearest sign-change, sample-on-zero, equidistant-tie-to-lower,
 // no-crossing keeps target, target clamp, degenerate buffers); waveformSurface (two stacked
@@ -309,6 +310,57 @@ static void testMarkerGrabInMonoSpansTheBand() {
     CHECK(markerAtPoint(s.overlay, frames, markers, 1, mx, b.bottom() + 5) == -1);
 }
 
+// --- The marker grab handle ----------------------------------------------------
+
+static void testMarkerHandleIsATopStripCentredOnTheMarker() {
+    const Rect a = wideArea();
+    const int mx = frameToX(overlayOf(a), 1000, 250);
+    const Rect h = markerHandleRect(overlayOf(a), 1000, 250);
+    CHECK(h.x == mx - kMarkerHandleHalfWidth);
+    CHECK(h.right() == mx + kMarkerHandleHalfWidth + 1);
+    CHECK(h.y == a.y);
+    CHECK(h.height == kMarkerHandleHeight);
+    CHECK(contains(h, mx, a.y));
+    CHECK(contains(h, mx, a.y + kMarkerHandleHeight - 1));
+    CHECK(!contains(h, mx, a.y + kMarkerHandleHeight));  // below the strip is the column's
+}
+
+// The whole reason the handle exists: two markers that share a frame both stay reachable —
+// markerAtPoint gives its full-height column to the first in draw order, and the handle owns
+// the strip above. Without the split, the loser could never be dragged apart again.
+static void testCoincidentMarkersStayIndependentlyGrabbable() {
+    const Rect a = wideArea();
+    const std::int64_t markers[2] = {250, 250};
+    const int mx = frameToX(overlayOf(a), 1000, 250);
+    // The column resolves to the first marker at every height, including the top strip.
+    CHECK(markerAtPoint(overlayOf(a), 1000, markers, 2, mx, a.y) == 0);
+    CHECK(markerAtPoint(overlayOf(a), 1000, markers, 2, mx, a.bottom() - 1) == 0);
+    // The handle, asked first, resolves the second one in that same top strip.
+    CHECK(contains(markerHandleRect(overlayOf(a), 1000, 250), mx, a.y));
+    CHECK(!contains(markerHandleRect(overlayOf(a), 1000, 250), mx, a.bottom() - 1));
+}
+
+static void testMarkerHandleClipsIntoTheArea() {
+    const Rect a = wideArea();
+    // At the last frame the marker maps to right(); an unclipped tab would claim pixels
+    // outside the band the caller already hit-tested.
+    const Rect hi = markerHandleRect(overlayOf(a), 1000, 1000);
+    CHECK(hi.right() == a.right());
+    CHECK(!contains(hi, a.right(), a.y));
+    CHECK(contains(hi, a.right() - 1, a.y));
+    // And at frame 0 it cannot reach left of the band.
+    const Rect lo = markerHandleRect(overlayOf(a), 1000, 0);
+    CHECK(lo.x == a.x);
+    CHECK(!contains(lo, a.x - 1, a.y));
+}
+
+static void testMarkerHandleOnDegenerateAreas() {
+    CHECK(markerHandleRect(overlayOf(Rect{}), 1000, 0).empty());
+    // A band shorter than the strip yields a handle the height of the band, never taller.
+    const Rect thin = Rect{0, 0, 100, 4};
+    CHECK(markerHandleRect(overlayOf(thin), 1000, 500).height == 4);
+}
+
 // --- Per-lane envelope content -------------------------------------------------
 
 static void testAsymmetricStereoLanesCarryDifferentContent() {
@@ -379,6 +431,11 @@ int main() {
     testMarkerGrabReachesTheLowerStereoLane();
     testMarkerGrabInMonoSpansTheBand();
 
+    testMarkerHandleIsATopStripCentredOnTheMarker();
+    testCoincidentMarkersStayIndependentlyGrabbable();
+    testMarkerHandleClipsIntoTheArea();
+    testMarkerHandleOnDegenerateAreas();
+
     testAsymmetricStereoLanesCarryDifferentContent();
     testLaneEnvelopeRejectsOutOfRangeLane();