Auto merge of #26074 - jdm:transition-fix, r=SimonSapin

Avoid infinitely looping CSS transitions.

This change addresses the long-standing issue of CSS transitions not ending appropriately. It does not fundamentally change the way we process transitions/animations.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #20379
- [x] There are tests for these changes

components/layout/animation.rs

@@ -110,6 +110,7 @@ pub fn update_animation_state<E>(
                     .unwrap();
             }
 
+            debug!("expiring animation for {:?}", running_animation);
             expired_animations
                 .entry(*key)
                 .or_insert_with(Vec::new)

components/style/animation.rs

@@ -425,6 +425,11 @@ pub fn start_transitions_if_applicable(
             // above.
             property_animation.update(Arc::get_mut(new_style).unwrap(), 0.0);
 
+            debug!(
+                "checking {:?} for matching end value",
+                possibly_expired_animations
+            );
+
             // Per [1], don't trigger a new transition if the end state for that
             // transition is the same as that of a transition that's already
             // running on the same node.
@@ -852,26 +857,18 @@ pub fn complete_expired_transitions(
     node: OpaqueNode,
     style: &mut Arc<ComputedValues>,
     context: &SharedStyleContext,
-) -> bool {
-    let had_animations_to_expire;
-    {
-        let all_expired_animations = context.expired_animations.read();
-        let animations_to_expire = all_expired_animations.get(&node);
-        had_animations_to_expire = animations_to_expire.is_some();
-        if let Some(ref animations) = animations_to_expire {
-            for animation in *animations {
-                debug!("Updating expired animation {:?}", animation);
-                // TODO: support animation-fill-mode
-                if let Animation::Transition(_, _, ref frame) = *animation {
-                    frame.property_animation.update(Arc::make_mut(style), 1.0);
-                }
+    expired_animations: &mut Vec<crate::animation::PropertyAnimation>,
+) {
+    let mut all_expired_animations = context.expired_animations.write();
+    if let Some(animations) = all_expired_animations.remove(&node) {
+        debug!("removing expired animations for {:?}", node);
+        for animation in animations {
+            debug!("Updating expired animation {:?}", animation);
+            // TODO: support animation-fill-mode
+            if let Animation::Transition(_, _, frame) = animation {
+                frame.property_animation.update(Arc::make_mut(style), 1.0);
+                expired_animations.push(frame.property_animation);
             }
         }
     }
-
-    if had_animations_to_expire {
-        context.expired_animations.write().remove(&node);
-    }
-
-    had_animations_to_expire
 }

components/style/matching.rs

@@ -607,7 +607,12 @@ trait PrivateMatchMethods: TElement {
 
         // Finish any expired transitions.
         let this_opaque = self.as_node().opaque();
-        animation::complete_expired_transitions(this_opaque, style, context);
+        animation::complete_expired_transitions(
+            this_opaque,
+            style,
+            context,
+            possibly_expired_animations,
+        );
 
         // Merge any running animations into the current style, and cancel them.
         let had_running_animations = context