diff --git a/crates/effects/src/distort.rs b/crates/effects/src/distort.rs
index a68de109..534da806 100644
--- a/crates/effects/src/distort.rs
+++ b/crates/effects/src/distort.rs
@@ -164,7 +164,10 @@ fn wave_warp(ctx: &EffectCtx, mut b: Buf) -> Buf {
     let kind = ctx.params.e("waveType");
     let pin = ctx.params.e("pinning");
     let seed = ctx.seed ^ 0x3a7e;
-    let (dx, dy) = (dir.sin(), -dir.cos());
+    // Cardinal directions must not acquire a tiny perpendicular slope: at a square-wave
+    // boundary cos(90°)'s rounding residue can flip a whole column on CPU but not GPU.
+    let axis = |v: f64| if v.abs() < 4.0 * f64::EPSILON { 0.0 } else { v };
+    let (dx, dy) = (axis(dir.sin()), -axis(dir.cos()));
     if !ctx.adjustment {
         b.pad(h.abs().ceil() as u32 + 1);
     }
@@ -556,8 +559,21 @@ mod tests {
             let (a, b) = (o.img.get(pad, y + pad), im.get(0, y));
             assert!((0..4).all(|i| (a[i] - b[i]).abs() < 0.01), "row {y}: {a:?} vs {b:?}");
         }
-        let free = run("ec.distort.wavewarp", &[("direction", num(0.0)), ("height", num(5.0))], im);
+        let free = run("ec.distort.wavewarp", &[("direction", num(0.0)), ("height", num(5.0))], im.clone());
         assert!(free.img.data.iter().zip(&o.img.data).any(|(a, b)| differs(*a, *b)));
+        // #1181: direction 90° → vertical displacement. Top Edge (4) holds the first row,
+        // Bottom Edge (6) the last, and the rest of the layer still moves.
+        for (pin, edge_row) in [(Value::Enum(4), 0i64), (Value::Enum(6), 29i64)] {
+            let set = [("direction", num(90.0)), ("height", num(5.0)), ("pinning", pin)];
+            let o = run("ec.distort.wavewarp", &set, im.clone());
+            let pad = ((o.img.width - im.width) / 2) as i64;
+            for x in 0..40 {
+                let (a, b) = (o.img.get(x + pad, edge_row + pad), im.get(x, edge_row));
+                assert!((0..4).all(|i| (a[i] - b[i]).abs() < 0.01), "pin {edge_row} col {x}: {a:?} vs {b:?}");
+            }
+            let mid = 15;
+            assert!((0..40).any(|x| differs(o.img.get(x + pad, mid + pad), im.get(x, mid))), "pin {edge_row}: the middle of the layer must still distort");
+        }
     }
 
     #[test]
diff --git a/crates/gpu/src/fx_distort.rs b/crates/gpu/src/fx_distort.rs
index a62ad683..ce713382 100644
--- a/crates/gpu/src/fx_distort.rs
+++ b/crates/gpu/src/fx_distort.rs
@@ -111,7 +111,10 @@ fn wave_warp(e: &mut Enc, ctx: &EffectCtx, mut b: GBuf) -> Option<GBuf> {
     let dir = ctx.params.f("direction").to_radians();
     let speed = ctx.params.f("speed");
     let phase = ctx.params.f("phase").to_radians() + ctx.time * speed * TAU;
-    let (dx, dy) = (dir.sin(), -dir.cos());
+    // Cardinal directions must not acquire a tiny perpendicular slope: at a square-wave
+    // boundary cos(90°)'s rounding residue can flip a whole column on CPU but not GPU.
+    let axis = |v: f64| if v.abs() < 4.0 * f64::EPSILON { 0.0 } else { v };
+    let (dx, dy) = (axis(dir.sin()), -axis(dir.cos()));
     if !ctx.adjustment {
         b.pad(e, h.abs().ceil() as u32 + 1)?;
     }
diff --git a/crates/gpu/src/tests_fx_distort.rs b/crates/gpu/src/tests_fx_distort.rs
index e0e89fb6..19cdcd60 100644
--- a/crates/gpu/src/tests_fx_distort.rs
+++ b/crates/gpu/src/tests_fx_distort.rs
@@ -28,6 +28,11 @@ fn twirl_bulge_ripple_wave_warp() {
     effect_case("ec.distort.wavewarp", &[("waveType", e(2)), ("pinning", e(1)), ("phase", n(400.0)), ("speed", n(-2.0))]);
     effect_case("ec.distort.wavewarp", &[("waveType", e(8)), ("pinning", e(2)), ("height", n(-5.0)), ("direction", n(200.0))]);
     effect_case("ec.distort.wavewarp", &[("waveType", e(4)), ("pinning", e(7)), ("width", n(33.0))]);
+    // #1181: the single-edge pinning paths, with the default vertical displacement direction.
+    effect_case("ec.distort.wavewarp", &[("direction", n(90.0)), ("height", n(7.0)), ("width", n(25.0)), ("pinning", e(4))]);
+    effect_case("ec.distort.wavewarp", &[("direction", n(90.0)), ("height", n(7.0)), ("width", n(19.0)), ("pinning", e(6)), ("waveType", e(1))]);
+    effect_case("ec.distort.wavewarp", &[("direction", n(45.0)), ("height", n(-6.0)), ("pinning", e(4)), ("speed", n(2.0))]);
+    effect_case("ec.distort.wavewarp", &[("direction", n(45.0)), ("height", n(-6.0)), ("pinning", e(6)), ("speed", n(2.0))]);
 }
 
 #[test]
@@ -79,6 +84,9 @@ fn shape_case(id: &str, vals: &[(&str, Value)]) {
 fn edge_pinning_on_a_shape_layer() {
     shape_case("ec.distort.turbulentdisplace", &[("size", n(30.0)), ("amount", n(20.0))]);
     shape_case("ec.distort.wavewarp", &[("pinning", e(1)), ("height", n(6.0)), ("width", n(25.0))]);
+    // #1181: single-edge pinning over the layer rect, not just All Edges.
+    shape_case("ec.distort.wavewarp", &[("pinning", e(4)), ("direction", n(90.0)), ("height", n(6.0)), ("width", n(25.0))]);
+    shape_case("ec.distort.wavewarp", &[("pinning", e(6)), ("direction", n(90.0)), ("height", n(6.0)), ("width", n(25.0))]);
     shape_case(
         "ec.distort.bulge",
         &[("pinAllEdges", on()), ("height", n(2.0)), ("hradius", n(40.0)), ("vradius", n(30.0)), ("center", Value::Vec2([28.5, 20.5]))],