fix: check custom element tag name when rebuilding (#4413)

tachys/src/html/element/mod.rs

@@ -317,6 +317,26 @@ where
     type State = ElementState<At::State, Ch::State>;
 
     fn rebuild(self, state: &mut Self::State) {
+        // check whether the tag is the same, for custom elements
+        // because this is const `false` for all other element types,
+        // the compiler should be able to optimize it out
+        if E::TAG.is_empty() {
+            // see https://github.com/leptos-rs/leptos/issues/4412
+            let new_tag = self.tag.tag();
+
+            // this is not particularly efficient, but it saves us from
+            // having to keep track of the tag name for every element state
+            let old_tag = state.el.tag_name();
+            if new_tag != old_tag {
+                let mut new_state = self.build();
+                state.insert_before_this(&mut new_state);
+                state.unmount();
+                *state = new_state;
+                return;
+            }
+        }
+
+        // rebuild attributes and children for any element
         let ElementState {
             attrs, children, ..
         } = state;