Fix waveform_view comment accuracy, cut duplicate comments, align xToFrame to OverlayArea

tests/test_waveform_view.cpp

@@ -26,8 +26,6 @@ static int g_fail = 0;
 #define CHECK(cond) do { if(!(cond)) { \
     std::printf("FAIL line %d: %s\n", __LINE__, #cond); ++g_fail; } } while(0)
 
-// frameToX/markerAtPoint/resolveDragFrame take the overlay type, not a bare Rect (the
-// distinct-type enforcement in editor_geometry.h) — this wraps a plain test Rect for them.
 static OverlayArea overlayOf(const Rect& r) { return OverlayArea{r}; }
 
 // A comfortable waveform area: 1000px wide, offset so left != 0 (catches origin bugs).
@@ -57,16 +55,16 @@ static void testFrameToXDegenerate() {
 
 static void testXToFrameInverse() {
     const Rect a = wideArea();
-    CHECK(xToFrame(a, 1000, a.x) == 0);
-    CHECK(xToFrame(a, 1000, a.right()) == 1000);
-    CHECK(xToFrame(a, 1000, a.x + 250) == 250);   // 1:1 map here
+    CHECK(xToFrame(overlayOf(a), 1000, a.x) == 0);
+    CHECK(xToFrame(overlayOf(a), 1000, a.right()) == 1000);
+    CHECK(xToFrame(overlayOf(a), 1000, a.x + 250) == 250);   // 1:1 map here
 }
 
 static void testXToFrameClampsOutside() {
     const Rect a = wideArea();
-    CHECK(xToFrame(a, 1000, a.x - 100) == 0);     // left of area -> 0
-    CHECK(xToFrame(a, 1000, a.right() + 100) == 1000); // right of area -> frameCount
-    CHECK(xToFrame(a, 0, a.x + 10) == 0);         // no frames -> 0
+    CHECK(xToFrame(overlayOf(a), 1000, a.x - 100) == 0);     // left of area -> 0
+    CHECK(xToFrame(overlayOf(a), 1000, a.right() + 100) == 1000); // right of area -> frameCount
+    CHECK(xToFrame(overlayOf(a), 0, a.x + 10) == 0);         // no frames -> 0
 }
 
 static void testFrameToXRoundTrip() {
@@ -75,7 +73,7 @@ static void testFrameToXRoundTrip() {
     const Rect a = Rect::ltrb(0, 0, 800, 60);
     for (std::int64_t f = 0; f <= 2000; f += 137) {
         const int x = frameToX(overlayOf(a), 2000, f);
-        const std::int64_t back = xToFrame(a, 2000, x);
+        const std::int64_t back = xToFrame(overlayOf(a), 2000, x);
         CHECK(back >= f - 3 && back <= f + 3);
     }
 }
@@ -269,6 +267,15 @@ static void testSurfaceDegenerateBandDrawsNothing() {
     CHECK(waveformOverlayArea(Rect{10, 10, 0, 0}).rect.empty());
 }
 
+static void testSurfaceThinBandRoundsLowerLaneEmpty() {
+    // Height 3 is the edge where the stereo split's integer division rounds the lower lane to
+    // empty even though the band itself isn't degenerate — pins the laneCount derivation.
+    const WaveformSurface s = waveformSurface(Rect{0, 0, 100, 3}, true, 2);
+    CHECK(s.laneCount == 1);
+    CHECK(!s.upper.empty());
+    CHECK(s.lower.empty());
+}
+
 // --- Hit-testing across the stacked lanes -------------------------------------
 
 static void testMarkerGrabReachesTheLowerStereoLane() {
@@ -367,6 +374,7 @@ int main() {
     testSurfaceMonoSourceInStereoModeStaysOneLane();
     testSurfaceOverlayIsFullStackedHeightInBothModes();
     testSurfaceDegenerateBandDrawsNothing();
+    testSurfaceThinBandRoundsLowerLaneEmpty();
 
     testMarkerGrabReachesTheLowerStereoLane();
     testMarkerGrabInMonoSpansTheBand();