Expose live computed CSS variables to scripts

crates/obscura-render/src/css.rs

@@ -955,7 +955,9 @@ impl Stylesheet {
         }
         let registration_changed_props = resolved_props != *parent_props;
         let has_own = !own.is_empty();
+        let mut own_names = std::collections::HashSet::new();
         for (k, v) in own {
+            own_names.insert(k.clone());
             let registration = self.registered_custom_properties.get(&k);
             let set_initial = |props: &mut HashMap<String, String>| {
                 if let Some(initial) = registration.and_then(|entry| entry.initial_value.as_ref()) {
@@ -997,6 +999,29 @@ impl Stylesheet {
                 }
             }
         }
+        // Custom properties inherit their computed value, not their original
+        // token stream. Resolve only declarations won on this element against
+        // the complete same-element environment (so forward references work),
+        // then pass those substituted values to descendants. Re-resolving an
+        // inherited `--b:var(--a)` after a child overrides `--a` is observably
+        // wrong: browsers keep the parent's already-computed `--b`.
+        let resolution_environment = resolved_props.clone();
+        for name in own_names {
+            let Some(value) = resolution_environment.get(&name) else {
+                continue;
+            };
+            if let Some(computed) = substitute_var_value(value, &resolution_environment, 0) {
+                resolved_props.insert(name, computed);
+            } else if let Some(initial) = self
+                .registered_custom_properties
+                .get(&name)
+                .and_then(|entry| entry.initial_value.clone())
+            {
+                resolved_props.insert(name, initial);
+            } else {
+                resolved_props.remove(&name);
+            }
+        }
         let effective = if has_own || registration_changed_props {
             Some(resolved_props)
         } else {
@@ -1419,7 +1444,11 @@ fn substitute_declarations(css: &str, props: &HashMap<String, String>) -> String
 /// represents CSS's guaranteed-invalid value. Crucially, invalidity propagates
 /// through an intermediate custom property so an outer var() can use its own
 /// fallback (`--toggle:var(--missing) dark; color:var(--toggle,light)`).
-fn substitute_var_value(input: &str, props: &HashMap<String, String>, depth: u8) -> Option<String> {
+pub(crate) fn substitute_var_value(
+    input: &str,
+    props: &HashMap<String, String>,
+    depth: u8,
+) -> Option<String> {
     if depth > 16 {
         return None;
     }

crates/obscura-render/src/dom.rs

@@ -225,6 +225,13 @@ impl OverflowClip {
 pub struct DomLayout {
     pub rects: HashMap<NodeId, Rect>,
     pub styles: HashMap<NodeId, crate::LayoutStyle>,
+    /// Computed custom properties in force for each element. Values are
+    /// reference-counted because the overwhelmingly common case is inheritance
+    /// without an override: an entire subtree can share one cascade map while
+    /// CSSOM still exposes every inherited `--token` through
+    /// `getComputedStyle()`.
+    pub custom_properties:
+        HashMap<NodeId, std::rc::Rc<HashMap<String, String>>>,
     /// The per-axis clip inherited from ancestor non-visible overflow, keyed
     /// per node, in SCREEN space (the clip owner's box shifted by the owner's
     /// accumulated translate; see `resolve_clip_rects`). `None` means
@@ -1348,6 +1355,7 @@ fn cascade_walk(
     sheet: &crate::css::Stylesheet,
     matcher: &mut obscura_dom::selector::Matcher,
     styles: &mut HashMap<NodeId, crate::LayoutStyle>,
+    custom_properties: &mut HashMap<NodeId, std::rc::Rc<HashMap<String, String>>>,
     parent_props: &std::rc::Rc<HashMap<String, String>>,
     container_evaluator: &mut Option<&mut crate::css::ContainerQueryEvaluator<'_>>,
     quirks_mode: bool,
@@ -1471,6 +1479,7 @@ fn cascade_walk(
         if let Some(m) = effective_props {
             this_props = std::rc::Rc::new(m);
         }
+        custom_properties.insert(id, this_props.clone());
         let (before_pseudo, after_pseudo) =
             if let Some(evaluator) = container_evaluator.as_deref_mut() {
                 sheet.pseudo_styles_with_container_queries(
@@ -1508,6 +1517,7 @@ fn cascade_walk(
             sheet,
             matcher,
             styles,
+            custom_properties,
             &this_props,
             container_evaluator,
             quirks_mode,
@@ -1960,6 +1970,7 @@ fn layout_dom_once(
     let t1 = std::time::Instant::now();
     let mut matcher = tree.matcher();
     let mut styles: HashMap<NodeId, crate::LayoutStyle> = HashMap::new();
+    let mut custom_properties = HashMap::new();
     let root_props = std::rc::Rc::new(HashMap::new());
     let mut evaluator =
         snapshot.map(|snapshot| crate::css::ContainerQueryEvaluator::new(tree, snapshot));
@@ -1980,6 +1991,7 @@ fn layout_dom_once(
         &sheet,
         &mut matcher,
         &mut styles,
+        &mut custom_properties,
         &root_props,
         &mut evaluator_ref,
         quirks_mode,
@@ -3948,6 +3960,7 @@ fn layout_dom_once(
         DomLayout {
             rects,
             styles,
+            custom_properties,
             clip_rects,
             translates,
             text_runs,

crates/obscura-render/src/paint.rs

@@ -461,6 +461,26 @@ impl PreparedRender {
             css_color(style.background_color.unwrap_or([0, 0, 0, 0])),
         );
         out.insert("color", css_color(style.color.unwrap_or([0, 0, 0, 255])));
+        out.insert("font-size", css_px(style.font_size.unwrap_or(16.0)));
+        out.insert(
+            "font-weight",
+            style
+                .font_weight
+                .clone()
+                .unwrap_or_else(|| "400".to_string()),
+        );
+        if let Some(family) = &style.font_family {
+            out.insert("font-family", family.clone());
+        }
+        out.insert(
+            "line-height",
+            match style.line_height.unwrap_or(crate::LineHeight::Normal) {
+                crate::LineHeight::Normal => "normal".to_string(),
+                crate::LineHeight::Ratio(value) => css_number(value),
+                crate::LineHeight::Px(value) => css_px(value),
+                crate::LineHeight::Relative(value) => dimension_css(value, "normal"),
+            },
+        );
 
         if let Some(rect) = rect {
             let horizontal_non_content =
@@ -621,6 +641,18 @@ impl PreparedRender {
         Some(out)
     }
 
+    /// Cascaded custom properties exposed by CSSOM alongside the compact
+    /// fixed-property snapshot above. These maps are shared across unchanged
+    /// inherited subtrees by `DomLayout`, so reading one does not require a
+    /// second cascade or duplicate every design token for every descendant.
+    pub fn computed_custom_properties(
+        &self,
+        id: obscura_dom::tree::NodeId,
+    ) -> Option<HashMap<String, String>> {
+        let properties = self.layout.custom_properties.get(&id)?;
+        Some(properties.as_ref().clone())
+    }
+
     /// Border box in the current root viewport. This is the read-only geometry
     /// path used by a later CSSOM integration and shares paint's clamped scroll,
     /// fixed-subtree, and sticky-positioning derivatives.