3 files changed, 30 insertions, 2 deletions
diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index 8b66d2db75d..cf63dfcfb9a 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -283,6 +283,12 @@ pub struct Document {
     last_click_info: DOMRefCell<Option<(Instant, Point2D<f32>)>>,
     /// https://html.spec.whatwg.org/multipage/#ignore-destructive-writes-counter
     ignore_destructive_writes_counter: Cell<u32>,
+    /// Track the total number of elements in this DOM's tree.
+    /// This is sent to the layout thread every time a reflow is done;
+    /// layout uses this to determine if the gains from parallel layout will be worth the overhead.
+    ///
+    /// See also: https://github.com/servo/servo/issues/10110
+    dom_count: Cell<u32>,
 }
 
 #[derive(JSTraceable, HeapSizeOf)]
@@ -454,6 +460,22 @@ impl Document {
         self.base_element.set(base.r());
     }
 
+    pub fn dom_count(&self) -> u32 {
+        self.dom_count.get()
+    }
+
+    /// This is called by `bind_to_tree` when a node is added to the DOM.
+    /// The internal count is used by layout to determine whether to be sequential or parallel.
+    /// (it's sequential for small DOMs)
+    pub fn increment_dom_count(&self) {
+        self.dom_count.set(self.dom_count.get() + 1);
+    }
+
+    /// This is called by `unbind_from_tree` when a node is removed from the DOM.
+    pub fn decrement_dom_count(&self) {
+        self.dom_count.set(self.dom_count.get() - 1);
+    }
+
     pub fn quirks_mode(&self) -> QuirksMode {
         self.quirks_mode.get()
     }
@@ -1880,6 +1902,7 @@ impl Document {
             target_element: MutNullableHeap::new(None),
             last_click_info: DOMRefCell::new(None),
             ignore_destructive_writes_counter: Default::default(),
+            dom_count: Cell::new(1),
         }
     }
 
diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index c384001e33a..6df2122e7b3 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -2151,10 +2151,12 @@ impl VirtualMethods for Element {
             return;
         }
 
+        let doc = document_from_node(self);
         if let Some(ref value) = *self.id_attribute.borrow() {
-            let doc = document_from_node(self);
             doc.register_named_element(self, value.clone());
         }
+        // This is used for layout optimization.
+        doc.increment_dom_count();
     }
 
     fn unbind_from_tree(&self, context: &UnbindContext) {
@@ -2164,10 +2166,12 @@ impl VirtualMethods for Element {
             return;
         }
 
+        let doc = document_from_node(self);
         if let Some(ref value) = *self.id_attribute.borrow() {
-            let doc = document_from_node(self);
             doc.unregister_named_element(self, value.clone());
         }
+        // This is used for layout optimization.
+        doc.decrement_dom_count();
     }
 
     fn children_changed(&self, mutation: &ChildrenMutation) {
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index e5e85051c7c..3b1bd124398 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -1083,6 +1083,7 @@ impl Window {
             window_size: window_size,
             script_join_chan: join_chan,
             query_type: query_type,
+            dom_count: self.Document().dom_count(),
         };
 
         self.layout_chan.send(Msg::Reflow(reflow)).unwrap();