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

src/core/instrument/ui/waveform_view.cpp

@@ -67,6 +67,18 @@ std::int64_t xToFrame(const OverlayArea& area, std::int64_t frameCount, int x) {
     return clampFrame(num / static_cast<std::int64_t>(w), frameCount);
 }
 
+Rect markerHandleRect(const OverlayArea& area, std::int64_t frameCount, std::int64_t frame) {
+    const Rect& r = area.rect;
+    if (r.empty()) return Rect{};
+    const int mx = frameToX(area, frameCount, frame);
+    // Clip into the area: a marker at the last frame maps to right(), whose unclipped tab
+    // would claim pixels outside the band the caller already hit-tested.
+    const int left = std::max(r.x, mx - kMarkerHandleHalfWidth);
+    const int right = std::min(r.right(), mx + kMarkerHandleHalfWidth + 1);
+    if (right <= left) return Rect{};
+    return Rect{left, r.y, right - left, std::min(kMarkerHandleHeight, r.height)};
+}
+
 int markerAtPoint(const OverlayArea& area, std::int64_t frameCount, const std::int64_t* frames,
                   int count, int x, int y) {
     if (count <= 0 || frames == nullptr) return -1;

src/core/instrument/ui/waveform_view.h

@@ -65,6 +65,15 @@ int frameToX(const OverlayArea& area, std::int64_t frameCount, std::int64_t fram
 // area.x yields 0; right of area.right() yields frameCount.
 std::int64_t xToFrame(const OverlayArea& area, std::int64_t frameCount, int x);
 
+// A marker's grab HANDLE: a tab riding the top of the overlay, centred on the marker's x and
+// clipped into the area. Distinct from the full-height grab COLUMN markerAtPoint answers, so
+// two markers that share a frame stay independently grabbable — the handle owns the top
+// strip, the column owns everything below it. Without that split, first-in-draw-order wins
+// every coincident tie and the loser can never be dragged apart again.
+inline constexpr int kMarkerHandleHeight = 10;
+inline constexpr int kMarkerHandleHalfWidth = 5;
+Rect markerHandleRect(const OverlayArea& area, std::int64_t frameCount, std::int64_t frame);
+
 // Which marker (index into the caller's parallel `frames` array, in draw order) a grab at
 // (x, y) lands on, or -1 for a miss. A marker is grabbed when x is within kMarkerGrabWidth of
 // its drawn x and y is inside `area`. First marker in draw order wins an overlapping tie.