Keep a styled layer's fill variable through .fig round trips

Fixes #399

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

crates/format/src/fig/assets_tests.rs

@@ -450,6 +450,38 @@ fn shared_styles_and_their_links_round_trip() {
     assert_eq!(named(&nodes, "Title")["styleIdForText"]["guid"], named(&nodes, "Heading")["guid"]);
 }
 
+#[test]
+fn a_styled_layers_fill_variable_survives_a_round_trip() {
+    let mut doc = Document::new("Overlap");
+    let page = doc.first_page().unwrap();
+    let mode = |id: &str, name: &str| Mode { id: id.into(), name: name.into() };
+    doc.variables.collections =
+        vec![Collection { id: "c-theme".into(), name: "Theme".into(), modes: vec![mode("m-light", "Light"), mode("m-dark", "Dark")], extends: None }];
+    let light = Color::new(0.1, 0.3, 0.7, 1.0);
+    let dark = VariableValue::Color(Color::new(0.8, 0.9, 1.0, 1.0));
+    doc.variables.variables =
+        vec![var("v-accent", "Accent color", "c-theme", VariableType::Color, &[("m-light", VariableValue::Color(light)), ("m-dark", dark)])];
+    doc.styles = vec![Style {
+        id: "s-accent".into(),
+        name: "Accent".into(),
+        description: String::new(),
+        kind: StyleKind::Paint { paints: vec![Paint::solid(light)] },
+        bound: Default::default(),
+    }];
+    add(&mut doc, page, NodeKind::Rectangle, "Swatch", (0.0, 0.0), (120.0, 80.0), &|n| {
+        n.fills = vec![Paint::solid(light)];
+        n.styles.fill = Some("s-accent".into());
+        n.bound.insert("fill".into(), "v-accent".into());
+    });
+    let (back, _) = import(&export(&doc, None).unwrap().0, "x").unwrap();
+    let var = back.variables.variables.iter().find(|v| v.name == "Accent color").unwrap().id.clone();
+    let swatch = back.nodes().find(|n| n.name == "Swatch").unwrap();
+    let on_layer = swatch.bound.get("fill") == Some(&var) || swatch.bound.get("fills/0/color") == Some(&var);
+    let style = back.styles.iter().find(|s| Some(&s.id) == swatch.styles.fill.as_ref()).unwrap();
+    let on_style = style.bound.get("fills/0/color") == Some(&var);
+    assert!(on_layer || on_style, "the fill binding was lost: layer {:?}, style {:?}", swatch.bound, style.bound);
+}
+
 #[test]
 fn text_styles_on_part_of_a_text_are_written_as_values() {
     let mut doc = styled();

crates/format/src/fig/map.rs

@@ -492,13 +492,17 @@ impl<'a> Mapper<'a> {
         }
         let t = v.get("transform").and_then(matrix).unwrap_or_default();
         o.insert("relativeTransform".into(), rows(space * t));
-        // Paints, from the shared style when one is linked (the layer's own copy can be stale).
+        // Paints, from the shared style when one is linked (the layer's own copy can be stale),
+        // unless the layer's own paints are bound to variables: those bindings must not be lost.
         let mut styles = Obj::new();
+        let own = |k: &str| Some(v.list(k)).filter(|l| l.iter().any(paint_bound));
         let fill_style = self.style(v, "styleIdForFill", "inheritFillStyleID", "fill", &mut styles);
-        let fills = fill_style.map_or_else(|| v.list("fillPaints"), |s| s.get("fillPaints").map(Kv::list).unwrap_or_default());
+        let fills = own("fillPaints")
+            .unwrap_or_else(|| fill_style.map_or_else(|| v.list("fillPaints"), |s| s.get("fillPaints").map(Kv::list).unwrap_or_default()));
         o.insert("fills".into(), Value::Array(fills.iter().filter_map(paint).collect()));
         let stroke_style = self.style(v, "styleIdForStrokeFill", "inheritFillStyleIDForStroke", "stroke", &mut styles);
-        let strokes = stroke_style.map_or_else(|| v.list("strokePaints"), |s| s.get("fillPaints").map(Kv::list).unwrap_or_default());
+        let strokes = own("strokePaints")
+            .unwrap_or_else(|| stroke_style.map_or_else(|| v.list("strokePaints"), |s| s.get("fillPaints").map(Kv::list).unwrap_or_default()));
         o.insert("strokes".into(), Value::Array(strokes.iter().filter_map(paint).collect()));
         let effect_style = self.style(v, "styleIdForEffect", "inheritEffectStyleID", "effect", &mut styles);
         let effects = effect_style.map_or_else(|| v.list("effects"), |s| s.get("effects").map(Kv::list).unwrap_or_default());
@@ -1387,6 +1391,11 @@ fn child_layout(o: &mut Obj, v: &View, parent: Option<&View>) {
     }
 }
 
+/// Whether a paint's colour or one of its gradient stops is bound to a variable.
+fn paint_bound(p: &Kv) -> bool {
+    alias(p.get("colorVar")).is_some() || p.get("stopsVar").map(Kv::list).unwrap_or_default().iter().any(|s| alias(s.get("colorVar")).is_some())
+}
+
 fn paint(p: &Kv) -> Option<Value> {
     let ty = p.get("type").and_then(Kv::str)?;
     let mut o = Obj::new();