style: Fix propagation of text-decoration lines.

components/style/gecko/restyle_damage.rs

@@ -47,9 +47,10 @@ impl GeckoRestyleDamage {
     /// them, but Gecko has an interesting optimization when they mark accessed
     /// structs, so they effectively only diff structs that have ever been
     /// accessed from layout.
-    pub fn compute_style_difference(source: &nsStyleContext,
+    pub fn compute_style_difference(
-                                    new_style: &Arc<ComputedValues>)
+        source: &nsStyleContext,
-                                    -> StyleDifference {
+        new_style: &Arc<ComputedValues>
+    ) -> StyleDifference {
         // TODO(emilio): Const-ify this?
         let context = source as *const nsStyleContext as *mut nsStyleContext;
         let mut any_style_changed: bool = false;

components/style/matching.rs

@@ -98,15 +98,6 @@ pub enum ChildCascadeRequirement {
     MustCascadeDescendants,
 }
 
-impl From<StyleChange> for ChildCascadeRequirement {
-    fn from(change: StyleChange) -> ChildCascadeRequirement {
-        match change {
-            StyleChange::Unchanged => ChildCascadeRequirement::CanSkipCascade,
-            StyleChange::Changed => ChildCascadeRequirement::MustCascadeChildren,
-        }
-    }
-}
-
 bitflags! {
     /// Flags that represent the result of replace_rules.
     pub flags RulesChanged: u8 {
@@ -778,6 +769,8 @@ trait PrivateMatchMethods: TElement {
                              new_values: &Arc<ComputedValues>,
                              pseudo: Option<&PseudoElement>)
                              -> ChildCascadeRequirement {
+        use properties::computed_value_flags::*;
+
         // Don't accumulate damage if we're in a restyle for reconstruction.
         if shared_context.traversal_flags.for_reconstruct() {
             return ChildCascadeRequirement::MustCascadeChildren;
@@ -793,13 +786,26 @@ trait PrivateMatchMethods: TElement {
         let skip_applying_damage =
             restyle.reconstructed_self_or_ancestor();
 
-        let difference = self.compute_style_difference(&old_values,
+        let difference =
-                                                       &new_values,
+            self.compute_style_difference(&old_values, &new_values, pseudo);
-                                                       pseudo);
+
         if !skip_applying_damage {
             restyle.damage |= difference.damage;
         }
-        difference.change.into()
+
+        match difference.change {
+            StyleChange::Unchanged => {
+                // We need to cascade the children in order to ensure the
+                // correct propagation of text-decoration-line, which is a reset
+                // property.
+                if old_values.flags.contains(HAS_TEXT_DECORATION_LINE) !=
+                    new_values.flags.contains(HAS_TEXT_DECORATION_LINE) {
+                    return ChildCascadeRequirement::MustCascadeChildren;
+                }
+                ChildCascadeRequirement::CanSkipCascade
+            },
+            StyleChange::Changed => ChildCascadeRequirement::MustCascadeChildren,
+        }
     }
 
     /// Computes and applies restyle damage unless we've already maxed it out.
@@ -813,7 +819,10 @@ trait PrivateMatchMethods: TElement {
                              -> ChildCascadeRequirement {
         let difference = self.compute_style_difference(&old_values, &new_values, pseudo);
         restyle.damage |= difference.damage;
-        difference.change.into()
+        match difference.change {
+            StyleChange::Changed => ChildCascadeRequirement::MustCascadeChildren,
+            StyleChange::Unchanged => ChildCascadeRequirement::CanSkipCascade,
+        }
     }
 
     #[cfg(feature = "servo")]