Avoid recursion in clear_descendant_data.

MozReview-Commit-ID: LeMhhcKoLr9

components/style/traversal.rs

@@ -509,7 +509,7 @@ where
         if data.styles.is_display_none() {
             debug!("{:?} style is display:none - clearing data from descendants.",
                    element);
-            clear_descendant_data(element)
+            unsafe { clear_descendant_data(element); }
         }
 
         // Inform any paint worklets of changed style, to speculatively
@@ -829,26 +829,30 @@ where
     }
 }
 
-/// Clear style data for all the subtree under `el`.
-pub fn clear_descendant_data<E>(el: E)
+/// Clear style data for all the subtree under `root` (but not for root itself).
+///
+/// We use a list to avoid unbounded recursion, which we need to avoid in the
+/// parallel traversal because the rayon stacks are small.
+pub unsafe fn clear_descendant_data<E>(root: E)
 where
     E: TElement,
 {
-    for kid in el.as_node().traversal_children() {
-        if let Some(kid) = kid.as_element() {
-            // We maintain an invariant that, if an element has data, all its
-            // ancestors have data as well.
-            //
-            // By consequence, any element without data has no descendants with
-            // data.
-            if kid.get_data().is_some() {
-                unsafe { kid.clear_data() };
-                clear_descendant_data(kid);
+    let mut parents = SmallVec::<[E; 32]>::new();
+    parents.push(root);
+    while let Some(p) = parents.pop() {
+        for kid in p.as_node().traversal_children() {
+            if let Some(kid) = kid.as_element() {
+                // We maintain an invariant that, if an element has data, all its
+                // ancestors have data as well.
+                //
+                // By consequence, any element without data has no descendants with
+                // data.
+                if kid.get_data().is_some() {
+                    kid.clear_data();
+                    parents.push(kid);
+                }
             }
         }
-    }
-
-    unsafe {
-        el.clear_descendants_bits();
+        p.clear_descendants_bits();
     }
 }