gpui: Add ParentElement impl for AnimationElement (#54145)

Hello, I've been loving using GPUI! Recently I noticed that calling
`.with_animiation()` would give an `AnimationElement<E>` which did not
allow `.child()` to be called on it. It was a quick fix, I hope it's a
quick easy merge but let me know if you'd like anything changed :)

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

Co-authored-by: Lukas Wirth <lukas@zed.dev>

crates/gpui/src/elements/animation.rs

@@ -2,7 +2,8 @@ use scheduler::Instant;
 use std::{rc::Rc, time::Duration};
 
 use crate::{
-    AnyElement, App, Element, ElementId, GlobalElementId, InspectorElementId, IntoElement, Window,
+    AnyElement, App, Element, ElementId, GlobalElementId, InspectorElementId, IntoElement,
+    ParentElement, Window,
 };
 
 pub use easing::*;
@@ -95,6 +96,16 @@ pub struct AnimationElement<E> {
     animator: Box<dyn Fn(E, usize, f32) -> E + 'static>,
 }
 
+impl<E: ParentElement> ParentElement for AnimationElement<E> {
+    fn extend(&mut self, elements: impl IntoIterator<Item = AnyElement>) {
+        let Some(element) = &mut self.element else {
+            return;
+        };
+
+        element.extend(elements);
+    }
+}
+
 impl<E> AnimationElement<E> {
     /// Returns a new [`AnimationElement<E>`] after applying the given function
     /// to the element being animated.
@@ -259,3 +270,33 @@ mod easing {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::InteractiveElement;
+    use crate::div;
+
+    use super::*;
+
+    // Before parent-animation-element, using .with_animation
+    // would not allow chaining .parent after. This is just a
+    // build check that we can call div().id().with_animation().child()
+    #[test]
+    fn test_animation_parent() {
+        div()
+            .id("id")
+            //
+            .with_animation(
+                "animation",
+                Animation::new(Duration::from_secs(1)),
+                |el, _t| {
+                    //
+                    el
+                },
+            )
+            .child(
+                //
+                div(),
+            );
+    }
+}