fix: correctly track ancestors for AtIndex (closes #4385)

reactive_stores/src/iter.rs

@@ -109,6 +109,23 @@ where
     fn keys(&self) -> Option<KeyMap> {
         self.inner.keys()
     }
+
+    fn track_field(&self) {
+        let mut full_path = self.path().into_iter().collect::<StorePath>();
+        let trigger = self.get_trigger(self.path().into_iter().collect());
+        trigger.this.track();
+        trigger.children.track();
+
+        // tracks `this` for all ancestors: i.e., it will track any change that is made
+        // directly to one of its ancestors, but not a change made to a *child* of an ancestor
+        // (which would end up with every subfield tracking its own siblings, because they are
+        // children of its parent)
+        while !full_path.is_empty() {
+            full_path.pop();
+            let inner = self.get_trigger(full_path.clone());
+            inner.this.track();
+        }
+    }
 }
 
 impl<Inner, Prev> DefinedAt for AtIndex<Inner, Prev>