Auto merge of #18622 - hiikezoe:display-none-by-cssom, r=birtles

Handle display property change from 'none' only if there is restyle hint for SMIL

<!-- Please describe your changes on the following line: -->
https://bugzilla.mozilla.org/show_bug.cgi?id=1401801

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

components/style/matching.rs

@@ -186,6 +186,9 @@ trait PrivateMatchMethods: TElement {
         use context::DISPLAY_CHANGED_FROM_NONE_FOR_SMIL;
         use properties::longhands::display::computed_value as display;
 
+        debug_assert!(restyle_hints.intersects(RESTYLE_SMIL),
+                      "Should have restyle hint for SMIL");
+
         let display_changed_from_none = old_values.map_or(false, |old| {
             let old_display_style = old.get_box().clone_display();
             let new_display_style = new_values.get_box().clone_display();
@@ -219,10 +222,12 @@ trait PrivateMatchMethods: TElement {
         use context::UpdateAnimationsTasks;
 
         if context.shared.traversal_flags.for_animation_only() {
+            if restyle_hint.intersects(RESTYLE_SMIL) {
                 self.handle_display_change_for_smil_if_needed(context,
                                                               old_values.as_ref().map(|v| &**v),
                                                               new_values,
                                                               restyle_hint);
+            }
             return;
         }