fix: pin the waveform arbitration in a testable predicate, close round-4 review minors

Extracts resolveWaveformClaim (core/instrument/ui/spline_edit) so the shell's node/tab/marker click resolution is unit-tested directly, not just its input geometry; folds the staged-envelope node into it; fixes comment accuracy, a cost regression, and test fidelity issues.

src/core/instrument/engine/play_params.h

@@ -195,12 +195,10 @@ bool splineActive(const Play& p) {
            (p.filter.enabled && p.filterSpline.mode == EnvMode::Spline);
 }
 
-// The one enforcement of the rule above: forces `p` to Trigger the instant it goes
-// splineActive. Every path that can leave `p` in a state splineActive() would newly return true
-// for — resolving stored params toward the engine, or an editor control edit that can flip a
-// mode/enable toggle — routes through this, so none of them can reopen the Gate+spline hole
-// splineActive's own doc comment describes. Header-inline and allocation-free: play_params.h
-// sits on the per-voice-per-sample include path.
+// The one enforcement of splineActive's rule (see its doc above). Header-inline and
+// allocation-free: play_params.h sits on the per-voice-per-sample include path. Both callers —
+// resolvePlay (sample_map.cpp) and the editor's applyControl — route through here, so the two
+// cannot drift apart.
 template <class Play>
 void enforceGateUnavailableWhileDrawn(Play& p) {
     if (splineActive(p)) p.playMode = PlayMode::Trigger;

src/core/instrument/ui/CMakeLists.txt

@@ -24,10 +24,8 @@ reasampler_pure_library(waveform_view
     SOURCES waveform_view.cpp
     LINK PUBLIC editor_geometry peaks PRIVATE sample_bands)
 # sample_bands is linked directly here because the test exercises the lane metrics that
-# waveform_view does not re-export. velocity_curve is linked for the smallest-target-first
-# arbitration tests, which pin the geometric facts editor_input_waveform.cpp's node/tab/marker
-# resolution depends on (the shell itself has no test target).
-reasampler_test(waveform_view LINK waveform_view sample_bands velocity_curve)
+# waveform_view does not re-export.
+reasampler_test(waveform_view LINK waveform_view sample_bands)
 
 reasampler_pure_library(browser_scroll
     SOURCES browser_scroll.cpp
@@ -63,7 +61,12 @@ reasampler_test(deck_groups LINK deck_groups sample_bands)
 reasampler_pure_library(spline_edit
     SOURCES spline_edit.cpp
     LINK PUBLIC editor_geometry velocity_curve)
-reasampler_test(spline_edit LINK spline_edit)
+# waveform_view and sample_bands are linked for the test only: resolveWaveformClaim's
+# smallest-target-first tests build the real node/tab/marker geometry
+# editor_input_waveform.cpp's mouseDownWaveform composes (the shell that calls it has no test
+# target of its own), which needs waveform_view's marker/tab primitives and sample_bands'
+# kWaveformMinHeight floor.
+reasampler_test(spline_edit LINK spline_edit waveform_view sample_bands)
 
 reasampler_pure_library(curve_popup SOURCES curve_popup.cpp LINK PUBLIC editor_geometry)
 # velocity_curve is linked for the test only: the sheet's geometry is domain-agnostic, and

src/core/instrument/ui/spline_edit.cpp

@@ -26,4 +26,14 @@ VelocityCurve::Box splineOverlayBox(const OverlayArea& area) {
     return VelocityCurve::Box{area.rect.x, area.rect.y, area.rect.width, area.rect.height};
 }
 
+WaveformClaimant resolveWaveformClaim(const WaveformClaim& node, const WaveformClaim& tab,
+                                      const WaveformClaim& marker, SplineGesture gesture) {
+    if (gesture == SplineGesture::kControlLeft && node.hit) return WaveformClaimant::kNode;
+    if (node.hit && (!tab.hit || node.area <= tab.area) && (!marker.hit || node.area <= marker.area))
+        return WaveformClaimant::kNode;
+    if (tab.hit && (!marker.hit || tab.area <= marker.area)) return WaveformClaimant::kTab;
+    if (marker.hit) return WaveformClaimant::kMarker;
+    return WaveformClaimant::kNone;
+}
+
 }  // namespace reasampler::instrument::ui

src/core/instrument/ui/spline_edit.h

@@ -1,11 +1,13 @@
 // spline_edit.h — the point-editing grammar's CLICK resolution: add/grab/delete/toggle from a
-// single (x, y). Both spline consumers — the velocity-curve popup and the spline EG overlay —
-// route their mouse-down through it, so the two cannot drift into two click grammars. Two more
-// gesture rules complete the grammar but live in the shell (see the note near the bottom of this
-// file). Mirror of envelope_edit otherwise: decision logic only, no host types, no drawing.
+// single (x, y), plus resolveWaveformClaim, the waveform overlay's cross-affordance arbitration
+// (node vs. crossfade tab vs. marker). Both spline consumers — the velocity-curve popup and the
+// spline EG overlay — route their mouse-down through the click grammar, so the two cannot drift
+// apart. Decision logic only, no host types, no drawing; mirror of envelope_edit otherwise.
 
 #pragma once
 
+#include <cstdint>
+
 #include "core/instrument/engine/velocity_curve.h"
 #include "core/instrument/ui/editor_geometry.h"  // Rect / OverlayArea
 
@@ -51,4 +53,28 @@ VelocityCurve::Box splineOverlayBox(const OverlayArea& area);
 //   strictly in-box, since splineOverlayBox has no inset — editor_input_waveform.cpp's
 //   splineOverlayClick.
 
+// One arbitration candidate: whether the affordance was hit under the cursor, and its own
+// pick-target area (nominal, per its own module's constants — not the actual clipped pixel
+// count; see resolveWaveformClaim).
+struct WaveformClaim {
+    bool hit = false;
+    std::int64_t area = 0;
+};
+
+// Which affordance a waveform-overlay click claims.
+enum class WaveformClaimant { kNone, kNode, kTab, kMarker };
+
+// The overlay's cross-affordance arbitration: a contour node (or, mutually exclusively, a
+// staged envelope's drag node — both feed the same `node` slot), the loop crossfade tab, and a
+// marker's full-height column can all claim the same pixel. Hit gates a candidate out
+// entirely; among the ones that hit, the SMALLEST nominal area wins — the marker column is the
+// odd one out (its target is the whole overlay height), so it only wins where nothing narrower
+// also claims the click. Ties go to whichever is checked first: node, then tab, then marker —
+// no live geometry produces a tie except tab-vs-marker, which the tab correctly wins (see
+// editor_input_waveform.cpp's mouseDownWaveform for the live constants). A control-click has no
+// tab/marker meaning (they answer plain grabs only), so it resolves to the node whenever the
+// node is in the running, regardless of area.
+WaveformClaimant resolveWaveformClaim(const WaveformClaim& node, const WaveformClaim& tab,
+                                      const WaveformClaim& marker, SplineGesture gesture);
+
 }  // namespace reasampler::instrument::ui