Run custom element upgrades through real constructors

crates/obscura-js/js/bootstrap.js

@@ -1035,6 +1035,13 @@ function _eventTargetDispatch(target, event) {
   return !event.defaultPrevented;
 }
 
+// During custom-element upgrade, HTMLElement's constructor must return the
+// already-existing element being upgraded. A class constructor cannot be
+// invoked with `.call(existingElement)`, so the registry and Element
+// constructor coordinate through the same construction-stack shape used by
+// browser custom-element implementations.
+const _customElementConstructionStack = [];
+
 class Node {
   static ELEMENT_NODE = 1;
   static ATTRIBUTE_NODE = 2;
@@ -1746,7 +1753,21 @@ globalThis.NamedNodeMap = NamedNodeMap;
 
 class Element extends Node {
   constructor(nid) {
-    super(nid);
+    const entry = _customElementConstructionStack[_customElementConstructionStack.length - 1];
+    const matchesUpgrade = entry && new.target === entry.constructor;
+    const upgrading = matchesUpgrade && !entry.constructed ? entry.element : null;
+    super(upgrading ? upgrading._nid : nid);
+    if (matchesUpgrade && entry.constructed) {
+      throw new TypeError("Custom element is already being constructed");
+    }
+    if (upgrading) {
+      // Keep an already-constructed marker on the stack until the outer class
+      // constructor returns. Recursive `new`/`super` calls for the same
+      // definition must not steal the element currently being upgraded.
+      entry.constructed = true;
+      Object.setPrototypeOf(upgrading, new.target.prototype);
+      return upgrading;
+    }
     this._style = _styleProxy(new CSSStyleDeclaration(this));
   }
   // Element wrappers always back a nodeType-1 node (_wrap/_wrapEl only build an
@@ -3340,15 +3361,24 @@ class Document extends Node {
     return _makeXPathResult(type, _xpathFindNodes(expression, contextNode || this));
   }
   createElement(t) {
-    const el = _wrapEl(+_dom("create_element", t.toLowerCase()));
-    if (el && t.toLowerCase() === 'template') {
+    const localName = String(t).toLowerCase();
+    const el = _wrapEl(+_dom("create_element", localName));
+    if (el && localName === 'template') {
       el._templateContent = this.createDocumentFragment();
       el._templateContent._fragmentContext = 'template';
     }
+    const definition = globalThis.customElements?._registry?.get(localName);
+    if (el && definition) globalThis.customElements._upgradeElement(el, definition);
     return el;
   }
   createElementNS(ns, t) {
+    const namespace = ns == null ? null : String(ns);
+    if (namespace === "http://www.w3.org/1999/xhtml") {
       const el = this.createElement(t);
+      if (el) el._ns = namespace;
+      return el;
+    }
+    const el = _wrapEl(+_dom("create_element", String(t).toLowerCase()));
     if (el) el._ns = ns;
     return el;
   }
@@ -5920,25 +5950,28 @@ class CustomElementRegistry {
     if (el.__customUpgraded) return;
     el.__customUpgraded = true;
     try {
-      // Web Components spec: copy own props from the prototype onto the
-      // element. JS-side classes define behavior via methods on the
-      // prototype; we don't truly swap prototypes (Element is shared),
-      // so attach the prototype methods directly to the instance.
-      const proto = cls.prototype;
-      for (const key of Object.getOwnPropertyNames(proto)) {
-        if (key === 'constructor') continue;
-        const desc = Object.getOwnPropertyDescriptor(proto, key);
-        if (desc) Object.defineProperty(el, key, desc);
+      // Upgrade preserves object identity but installs the definition's
+      // prototype before running its class constructor. HTMLElement's
+      // constructor consumes this entry and returns `el`, so derived class
+      // fields and constructor-side state initialize on the real DOM wrapper.
+      const constructionEntry = { element: el, constructor: cls, constructed: false };
+      _customElementConstructionStack.push(constructionEntry);
+      let constructed;
+      try {
+        constructed = Reflect.construct(cls, []);
+      } finally {
+        const pending = _customElementConstructionStack.lastIndexOf(constructionEntry);
+        if (pending !== -1) _customElementConstructionStack.splice(pending, 1);
+      }
+      if (constructed !== el) {
+        throw new TypeError("Custom element constructor did not produce the element being upgraded");
       }
-      // Run constructor-side init on the element. Real custom elements
-      // run the class constructor, but Element instances aren't a `cls`
-      // subclass here; calling `.call(el)` runs whatever init logic the
-      // class defines without needing a new allocation.
-      try { cls.call(el); } catch (e) {}
       if (typeof el.connectedCallback === 'function' && globalThis.document?.contains?.(el)) {
         try { el.connectedCallback(); } catch (e) {}
       }
-    } catch (e) {}
+    } catch (e) {
+      el.__customUpgradeFailed = true;
+    }
   }
   get(name) { return this._registry.get(name); }
   getName(cls) {

crates/obscura-js/src/runtime.rs

@@ -1897,6 +1897,133 @@ mod tests {
         assert_eq!(ok, serde_json::json!("OK"));
     }
 
+    #[test]
+    fn custom_element_upgrade_runs_class_constructor_on_existing_element() {
+        let mut rt = setup_runtime(
+            r#"<html><body><svelte-like id="component"></svelte-like></body></html>"#,
+        );
+        let result = rt
+            .evaluate(
+                r#"
+                const before = document.getElementById("component");
+                class SvelteLike extends HTMLElement {
+                    constructor() {
+                        super();
+                        this.$$s = [];
+                        this.attachShadow({ mode: "open" });
+                    }
+                    connectedCallback() {
+                        for (const subscription of this.$$s) subscription();
+                        this.$$s.push(() => {});
+                        this.shadowRoot.textContent = "ready";
+                    }
+                }
+                customElements.define("svelte-like", SvelteLike);
+                return [
+                    document.getElementById("component") === before,
+                    before instanceof SvelteLike,
+                    before.constructor === SvelteLike,
+                    before.$$s.length,
+                    before.shadowRoot && before.shadowRoot.textContent
+                ];
+                "#,
+            )
+            .unwrap();
+        assert_eq!(
+            result,
+            serde_json::json!([
+                true,
+                true,
+                true,
+                1,
+                "ready"
+            ])
+        );
+    }
+
+    #[test]
+    fn create_element_synchronously_constructs_an_existing_definition() {
+        let mut rt = setup_runtime("<html><body></body></html>");
+        let result = rt
+            .evaluate(
+                r#"
+                const testStart = true;
+                class CreatedLater extends HTMLElement {
+                    constructor() {
+                        super();
+                        this.constructorState = ["initialized"];
+                        this.attachShadow({ mode: "open" });
+                        this.shadowRoot.textContent = "constructed";
+                    }
+                    connectedCallback() {
+                        this.constructorState.push("connected");
+                    }
+                }
+                customElements.define("created-later", CreatedLater);
+                const element = document.createElement("created-later");
+                const foreign = document.createElementNS(
+                    "http://www.w3.org/2000/svg", "created-later"
+                );
+                return [
+                    element instanceof CreatedLater,
+                    element.constructor === CreatedLater,
+                    element.localName,
+                    element.constructorState,
+                    element.shadowRoot && element.shadowRoot.textContent,
+                    element.isConnected,
+                    foreign instanceof CreatedLater
+                ];
+                "#,
+            )
+            .unwrap();
+        assert_eq!(
+            result,
+            serde_json::json!([
+                true,
+                true,
+                "created-later",
+                ["initialized"],
+                "constructed",
+                false,
+                false
+            ])
+        );
+    }
+
+    #[test]
+    fn throwing_custom_element_constructor_marks_upgrade_failed_without_connecting() {
+        let mut rt = setup_runtime(
+            r#"<html><body><throws-during-upgrade id="target"></throws-during-upgrade></body></html>"#,
+        );
+        let result = rt
+            .evaluate(
+                r#"
+                let constructorCalls = 0;
+                let connectedCalls = 0;
+                class ThrowsDuringUpgrade extends HTMLElement {
+                    constructor() {
+                        super();
+                        constructorCalls++;
+                        throw new Error("expected constructor failure");
+                    }
+                    connectedCallback() {
+                        connectedCalls++;
+                    }
+                }
+                customElements.define("throws-during-upgrade", ThrowsDuringUpgrade);
+                const element = document.getElementById("target");
+                customElements.upgrade(document);
+                return [
+                    constructorCalls,
+                    connectedCalls,
+                    element.__customUpgradeFailed === true
+                ];
+                "#,
+            )
+            .unwrap();
+        assert_eq!(result, serde_json::json!([1, 0, true]));
+    }
+
     #[test]
     fn test_document_title() {
         let mut rt = setup_runtime("<html><head><title>Test</title></head><body></body></html>");