diff --git a/crates/render/src/masks.rs b/crates/render/src/masks.rs
index 0cfdc0e7..980e6475 100644
--- a/crates/render/src/masks.rs
+++ b/crates/render/src/masks.rs
@@ -134,7 +134,10 @@ pub fn shapes(ctx: &EvalCtx, layer: &Layer) -> Vec<effectcraft_effects::MaskShap
 /// Returns whether any mask was applied.
 pub fn apply(ctx: &EvalCtx, layer: &Layer, buf: &mut Buf, blur: MaskBlur) -> bool {
     let Some(masks) = layer.masks() else { return false };
-    let list: Vec<_> = masks.groups().filter(|g| g.enabled && matches!(g.kind, GroupKind::Mask { mode, .. } if mode != MaskMode::None)).collect();
+    // An open mask (a path the Pen is still drawing) makes no transparency: the layer shows
+    // whole until the path is closed (After Effects; effects still use it as a path).
+    let closed = |g: &effectcraft_project::PropGroup| matches!(ctx.group_value(layer, g, "path"), Some(Value::Path(sp)) if sp.closed);
+    let list: Vec<_> = masks.groups().filter(|g| g.enabled && matches!(g.kind, GroupKind::Mask { mode, .. } if mode != MaskMode::None) && closed(g)).collect();
     if list.is_empty() {
         return false;
     }
diff --git a/crates/render/src/tests.rs b/crates/render/src/tests.rs
index 01f04131..b95e27b9 100644
--- a/crates/render/src/tests.rs
+++ b/crates/render/src/tests.rs
@@ -368,6 +368,26 @@ fn effects_see_layer_masks() {
     assert!(img.get(150, 50)[3] > 0.99, "mode None mask does not cut the layer");
 }
 
+/// An open mask (the Pen still drawing it) leaves the layer whole; closed, it masks (#290).
+#[test]
+fn open_masks_make_no_transparency() {
+    let render = |closed: bool| {
+        let (mut p, cid, comp) = setup();
+        let mut l = solid(&mut p, &comp, [1.0, 0.0, 0.0], 200, 100);
+        let path = ShapePath { closed, ..ShapePath::rect([50.0, 50.0], 40.0, 40.0) };
+        let mut next = p.next_id;
+        let m = build::mask(&mut Ids(&mut next), "Mask 1", path, MaskMode::Add, [255, 255, 0]);
+        p.next_id = next;
+        l.props.sub_mut("masks").unwrap().children.push(m.into());
+        p.comp_mut(cid).unwrap().layers.push(l);
+        render_frame(&p, cid, Tick::ZERO, 1.0)
+    };
+    let open = render(false);
+    assert!(open.get(50, 50)[3] > 0.99 && open.get(150, 50)[3] > 0.99, "the open mask cut the layer");
+    let closed = render(true);
+    assert!(closed.get(50, 50)[3] > 0.99 && closed.get(150, 50)[3] < 0.01, "the closed mask masks");
+}
+
 #[test]
 fn effects_read_layer_params() {
     let (mut p, cid, comp) = setup();