Pass through visited style for after change

When a transition update is needed during restyling, `get_after_change_style` is
called to re-cascade without transition rules, and this style is then used for
primary styling instead of the style just computed.

`get_after_change_style` used to ignore visited styles, so this meant that if a
transition update was needed, visited styles were effectively dropped.

To fix the crash here, we pass through the existing visited styles when
re-cascading as part of `get_after_change_style`.

MozReview-Commit-ID: 4HBQAdeJ38B

components/style/context.rs

@@ -297,6 +297,11 @@ impl CascadeInputs {
         self.visited_rules.take()
     }
 
+    /// Whether there are any visited values.
+    pub fn has_visited_values(&self) -> bool {
+        self.visited_values.is_some()
+    }
+
     /// Gets a reference to the visited computed values. Panic if the element
     /// does not have visited computed values.
     pub fn visited_values(&self) -> &Arc<ComputedValues> {

components/style/matching.rs

@@ -577,6 +577,12 @@ trait PrivateMatchMethods: TElement {
             }
         }
 
+        // If there were visited values to insert, ensure they do in fact exist
+        // inside the new values.
+        debug_assert!(!cascade_visited.visited_values_for_insertion() ||
+                      context.cascade_inputs().primary().has_visited_values() ==
+                      new_values.has_visited_style());
+
         // Set the new computed values.
         let primary_inputs = context.cascade_inputs_mut().primary_mut();
         cascade_visited.set_primary_values(&mut data.styles,
@@ -661,8 +667,9 @@ trait PrivateMatchMethods: TElement {
             return None;
         }
 
-        // This currently ignores visited styles, which seems acceptable,
+        // This currently passes through visited styles, if they exist.
-        // as existing browsers don't appear to transition visited styles.
+        // When fixing bug 868975, compute after change for visited styles as
+        // well, along with updating the rest of the animation processing.
         Some(self.cascade_with_rules(context.shared,
                                      &context.thread_local.font_metrics_provider,
                                      &without_transition_rules,
@@ -670,7 +677,7 @@ trait PrivateMatchMethods: TElement {
                                      CascadeTarget::Normal,
                                      CascadeVisitedMode::Unvisited,
                                      /* parent_info = */ None,
-                                     None))
+                                     primary_style.get_visited_style().cloned()))
     }
 
     #[cfg(feature = "gecko")]
@@ -716,6 +723,9 @@ trait PrivateMatchMethods: TElement {
         use context::{CASCADE_RESULTS, CSS_ANIMATIONS, CSS_TRANSITIONS, EFFECT_PROPERTIES};
         use context::UpdateAnimationsTasks;
 
+        // Bug 868975: These steps should examine and update the visited styles
+        // in addition to the unvisited styles.
+
         let mut tasks = UpdateAnimationsTasks::empty();
         if self.needs_animations_update(context, old_values.as_ref(), new_values) {
             tasks.insert(CSS_ANIMATIONS);

components/style/properties/gecko.mako.rs

@@ -171,6 +171,11 @@ impl ComputedValues {
         self.rules.as_ref().unwrap()
     }
 
+    /// Whether there is a visited style.
+    pub fn has_visited_style(&self) -> bool {
+        self.visited_style.is_some()
+    }
+
     /// Gets a reference to the visited style, if any.
     pub fn get_visited_style(&self) -> Option<<&Arc<ComputedValues>> {
         self.visited_style.as_ref()

components/style/properties/properties.mako.rs

@@ -1908,6 +1908,11 @@ impl ComputedValues {
         self.rules.as_ref().unwrap()
     }
 
+    /// Whether there is a visited style.
+    pub fn has_visited_style(&self) -> bool {
+        self.visited_style.is_some()
+    }
+
     /// Gets a reference to the visited style, if any.
     pub fn get_visited_style(&self) -> Option<<&Arc<ComputedValues>> {
         self.visited_style.as_ref()