M6: Sketch import: shape groups that are clipping masks clip

Sketch 51 and older files make masks from shape groups; the mask flag was
lost when the group became its shape or boolean operation, so a masked map
spilled out of its card.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/format/src/sketch/path.rs

@@ -143,6 +143,7 @@ pub(crate) fn shape_group(cx: &mut Cx, l: &Value, parent: Affine, depth: usize,
                 super::style::clear(m);
                 super::style::apply(cx, st, cw, ch, m);
             }
+            super::tree::mask(l, m);
         }
         return Some(node);
     }
@@ -166,6 +167,7 @@ pub(crate) fn shape_group(cx: &mut Cx, l: &Value, parent: Affine, depth: usize,
     let mut node = base(l, "BOOLEAN_OPERATION", parent, world, w, h, root);
     node.insert("booleanOperation".into(), json!(op));
     node.insert("children".into(), Value::Array(chain));
+    super::tree::mask(l, &mut node);
     if let Some(st) = l.get("style") {
         super::style::apply(cx, st, w, h, &mut node);
     }

crates/format/src/sketch/style_tests.rs

@@ -233,3 +233,18 @@ fn style_on_frames_and_text_kinds_untouched() {
     assert!(matches!(node(&doc, "Board").kind, NodeKind::Frame));
     assert_eq!(node(&doc, "Board").fills.len(), 1);
 }
+
+#[test]
+fn shape_group_masks_clip() {
+    // Sketch 51 and older: the mask is a shape group.
+    let inner = with(r("Outline"), "booleanOperation", json!(-1));
+    let mask = with(shape_group("Card", 0.0, 0.0, 100.0, 50.0, vec![inner]), "hasClippingMask", json!(true));
+    let a = rect("A", 0.0, 0.0, 300.0, 300.0);
+    let b = with(r("B"), "shouldBreakMaskChain", json!(true));
+    let (doc, _) = open(vec![group("G", 0.0, 0.0, 300.0, 300.0, vec![mask, a, b])]);
+    let m = node(&doc, "Card");
+    assert!(m.is_mask);
+    let holder = doc.node(m.parent.unwrap()).unwrap();
+    let kids: Vec<&str> = holder.children.iter().map(|c| doc.node(*c).unwrap().name.as_str()).collect();
+    assert_eq!(kids, ["Card", "A"]);
+}

crates/format/src/sketch/tree.rs

@@ -95,10 +95,7 @@ pub(crate) fn layer(cx: &mut Cx, l: &Value, parent: Affine, depth: usize, root:
         let shape = super::path::shape(cx, l, w, h)?;
         super::path::put_shape(cx, shape, &mut node);
     }
-    if bool_field(l, "hasClippingMask") == Some(true) {
-        node.insert("isMask".into(), json!(true));
-        node.insert("maskType".into(), json!(if l.get("clippingMaskMode").and_then(Value::as_i64) == Some(1) { "ALPHA" } else { "VECTOR" }));
-    }
+    mask(l, &mut node);
     match class {
         "artboard" | "frame" => {
             let clips = if class == "artboard" { true } else { bool_field(l, "clipsContent").unwrap_or(true) };
@@ -129,6 +126,14 @@ pub(crate) fn layer(cx: &mut Cx, l: &Value, parent: Affine, depth: usize, root:
     Some(Value::Object(node))
 }
 
+/// A clipping mask's flag and kind (outline or alpha).
+pub(crate) fn mask(l: &Value, node: &mut JsonMap<String, Value>) {
+    if bool_field(l, "hasClippingMask") == Some(true) {
+        node.insert("isMask".into(), json!(true));
+        node.insert("maskType".into(), json!(if l.get("clippingMaskMode").and_then(Value::as_i64) == Some(1) { "ALPHA" } else { "VECTOR" }));
+    }
+}
+
 /// Count one more layer; false (with a warning) once the file's budget is spent.
 fn count(cx: &mut Cx) -> bool {
     if cx.nodes >= cx.max_nodes {