style: Update animations when a pseudo-element had animations but no longer has, and has been re-framed in the meantime.

This is the easy fix.

The hard fix (outlined in the comment) would be nice, but I don't think this bug
alone justifies it.

Differential Revision: https://phabricator.services.mozilla.com/D38184

components/style/matching.rs

@@ -240,16 +240,31 @@ trait PrivateMatchMethods: TElement {
         let new_box_style = new_style.get_box();
         let new_style_specifies_animations = new_box_style.specifies_animations();
 
-        let old_style = match old_style {
-            Some(old) => old,
-            None => return new_style_specifies_animations,
-        };
-
         let has_animations = self.has_css_animations();
         if !new_style_specifies_animations && !has_animations {
             return false;
         }
 
+        let old_style = match old_style {
+            Some(old) => old,
+            // If we have no old style but have animations, we may be a
+            // pseudo-element which was re-created without style changes.
+            //
+            // This can happen when we reframe the pseudo-element without
+            // restyling it (due to content insertion on a flex container or
+            // such, for example). See bug 1564366.
+            //
+            // FIXME(emilio): The really right fix for this is keeping the
+            // pseudo-element itself around on reframes, but that's a bit
+            // harder. If we do that we can probably remove quite a lot of the
+            // EffectSet complexity though, since right now it's stored on the
+            // parent element for pseudo-elements given we need to keep it
+            // around...
+            None => {
+                return new_style_specifies_animations || new_style.is_pseudo_style();
+            }
+        };
+
         let old_box_style = old_style.get_box();
 
         let keyframes_could_have_changed = context

components/style/properties/gecko.mako.rs

@@ -117,9 +117,14 @@ impl ComputedValues {
         ).to_outer(None)
     }
 
+    #[inline]
+    pub fn is_pseudo_style(&self) -> bool {
+        self.0.mPseudoType != PseudoStyleType::NotPseudo
+    }
+
     #[inline]
     pub fn pseudo(&self) -> Option<PseudoElement> {
-        if self.0.mPseudoType == PseudoStyleType::NotPseudo {
+        if !self.is_pseudo_style() {
             return None;
         }
         PseudoElement::from_pseudo_type(self.0.mPseudoType)