Auto merge of #16938 - emilio:traversal-assert, r=bzbarsky

stylo: Don't mess up traversal drivers.

Right now it's harmless, but it trips assertions that are nice to have, just in
case it stops being the case.

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1 files changed, 2 insertions, 2 deletions

diff --git a/ports/geckolib/glue.rs b/ports/geckolib/glue.rs
index afbb8c7ef57..d04625727aa 100644
--- a/ports/geckolib/glue.rs
+++ b/ports/geckolib/glue.rs
@@ -210,14 +210,14 @@ fn traverse_subtree(element: GeckoElement,
     debug!("Traversing subtree:");
     debug!("{:?}", ShowSubtreeData(element.as_node()));
 
-    let traversal_driver = if global_style_data.style_thread_pool.is_none() {
+    let traversal_driver = if global_style_data.style_thread_pool.is_none() || !element.is_root() {
         TraversalDriver::Sequential
     } else {
         TraversalDriver::Parallel
     };
 
     let traversal = RecalcStyleOnly::new(shared_style_context, traversal_driver);
-    if traversal_driver.is_parallel() && element.is_root() {
+    if traversal_driver.is_parallel() {
         parallel::traverse_dom(&traversal, element, token,
                                global_style_data.style_thread_pool.as_ref().unwrap());
     } else {