Config: expose style traversal thread count

src/config.rs

@@ -82,6 +82,28 @@ pub struct Config {
 
     /// Background color as RGBA (default: white).
     pub background: [u8; 4],
+
+    /// Number of threads stylo uses for parallel style traversal.
+    ///
+    /// - `None` (default): use blitz's auto heuristic
+    ///   (`num_cpus * 3/4`, capped at 6).
+    /// - `Some(1)`: disable parallel traversal. Style work runs serially
+    ///   on the calling thread.
+    /// - `Some(n)` with `n >= 2`: use a pool of `n` worker threads.
+    ///
+    /// Set this to `Some(1)` when calling `render` concurrently from
+    /// multiple OS threads (e.g. a server rendering one document per
+    /// request via `tokio::task::spawn_blocking`). Stylo's
+    /// `STYLE_THREAD_POOL` is process-global; two parallel traversals
+    /// landing on the same rayon worker both try to mutably borrow the
+    /// worker's thread-local sharing cache and one panics. Disabling
+    /// parallelism keeps each render's stylo work on the caller's OS
+    /// thread, where the thread-local is uniquely owned.
+    ///
+    /// **One-shot:** stylo's `STYLE_THREAD_POOL` is initialised on first
+    /// access. The value set on the first `render(...)` call in the
+    /// process wins; later calls inherit that pool size regardless.
+    pub style_thread_count: Option<i32>,
 }
 
 impl Default for Config {
@@ -94,6 +116,7 @@ impl Default for Config {
             color_scheme: ColorScheme::Light,
             auto_height: false,
             background: [255, 255, 255, 255], // White
+            style_thread_count: None,
         }
     }
 }
@@ -248,6 +271,26 @@ impl Config {
         self.background([0, 0, 0, 0])
     }
 
+    /// Configure stylo's parallel style-traversal thread count.
+    ///
+    /// Pass `1` to disable parallel traversal when calling `render`
+    /// concurrently from multiple OS threads. See the field-level
+    /// docs on [`Config::style_thread_count`] for the full rationale.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// use hyper_render::Config;
+    ///
+    /// // Server context: one render per `spawn_blocking` thread.
+    /// let config = Config::new().style_thread_count(1);
+    /// assert_eq!(config.style_thread_count, Some(1));
+    /// ```
+    pub fn style_thread_count(mut self, count: i32) -> Self {
+        self.style_thread_count = Some(count);
+        self
+    }
+
     /// Minimum supported width/height in pixels.
     ///
     /// Very small dimensions can cause overflow issues in the underlying

src/lib.rs

@@ -166,6 +166,7 @@ fn create_document(html: &str, config: &Config) -> Result<HtmlDocument> {
 
     let doc_config = DocumentConfig {
         viewport: Some(viewport),
+        stylo_thread_count: config.style_thread_count,
         ..Default::default()
     };