Ω-W1-T5 review fixes: unify the drag frame resolve onto frameToX/xToFrame; honest comments on grabbableMarks and the grey-loop-mark contrast trade

tests/test_waveform_view.cpp

@@ -10,9 +10,10 @@
 // waveformOverlayArea (the overlay IS the drawn column band, inset symmetrically);
 // 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); the four marks (per-mark cap
+// resolveDragFrame (drag lands on the frameToX/xToFrame column under the cursor, 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); the four marks (per-mark cap
 // resolve, the reverse cap order that keeps a coincident pair separable, label sides/nudging,
 // the suppression rule and its promoted-first placement, the crossfade wedge ramp);
 // waveformSurface (two stacked
@@ -263,6 +264,36 @@ static void testResolveDragFrameDegenerate() {
     CHECK(resolveDragFrame(ov, 1000, 5000, 0) == 1000);
 }
 
+// A drag must land on the column under the cursor — the SAME frameToX/xToFrame partition, never
+// a proportional approximation of it. Pins the contract itself (grabX = frameToX(startFrame),
+// result = xToFrame(grabX + dxPixels)) rather than a captured number, in the frames < columns
+// regime where the two disagree: a prior independent linear map here left a marker at frame 10
+// (1000px/37 frames, start=10, +3px) when a fresh xToFrame(x) at the same cursor column
+// resolves to frame 11 — exactly the class of drift a second frame<->pixel map produces.
+static void testResolveDragFrameLandsOnCursorColumn() {
+    const OverlayArea ov = overlayOf(Rect::ltrb(0, 0, 1000, 60));
+    const std::int64_t frameCount = 37;
+    const std::int64_t startFrame = 10;
+    const int dx = 3;
+    const int grabX = frameToX(ov, frameCount, startFrame);
+    const std::int64_t cursorFrame = xToFrame(ov, frameCount, grabX + dx);
+    CHECK(cursorFrame == 11);                              // the contract's own derivation
+    CHECK(resolveDragFrame(ov, frameCount, startFrame, dx) == cursorFrame);
+    // General form, swept across both frames<columns and frames>columns: the resolved frame's
+    // OWN column (frameToX) must contain the cursor pixel the drag actually landed on.
+    const std::int64_t counts[] = {5, 37, 251, 9973};
+    const int deltas[] = {-97, -3, -1, 1, 3, 97};
+    for (std::int64_t n : counts) {
+        for (std::int64_t start = 0; start < n; start += (std::max<std::int64_t>)(1, n / 11)) {
+            for (int d : deltas) {
+                const std::int64_t got = resolveDragFrame(ov, n, start, d);
+                const int wantGrabX = frameToX(ov, n, start);
+                CHECK(got == xToFrame(ov, n, wantGrabX + d));
+            }
+        }
+    }
+}
+
 // --- nearestZeroCrossing ------------------------------------------------------
 
 static void testZeroCrossingNearest() {
@@ -786,6 +817,7 @@ int main() {
     testResolveDragFrameClamps();
     testResolveDragFrameRounds();
     testResolveDragFrameDegenerate();
+    testResolveDragFrameLandsOnCursorColumn();
 
     testZeroCrossingNearest();
     testZeroCrossingSampleOnZero();