[FIX] app: allow mounting owl apps in iframe

Previously, attempting to mount an app in an iframe would crash, saying
that the target is not a valid DOM element, this is because instanceof
checks do not work cross-frame as global objects do not have the same
identity in frames as with the main window. This commit fixes that by
making sure the target is an instance of HTMLElement of the
corresponding window, and checks that the corresponding document body
contains it.

src/runtime/utils.ts

@@ -28,12 +28,18 @@ export function batched(callback: Callback): Callback {
 }
 
 export function validateTarget(target: HTMLElement) {
-  if (!(target instanceof HTMLElement)) {
-    throw new OwlError("Cannot mount component: the target is not a valid DOM element");
-  }
-  if (!document.body.contains(target)) {
-    throw new OwlError("Cannot mount a component on a detached dom node");
+  // Get the document and HTMLElement corresponding to the target to allow mounting in iframes
+  const document = target && target.ownerDocument;
+  if (document) {
+    const HTMLElement = document.defaultView!.HTMLElement;
+    if (target instanceof HTMLElement) {
+      if (!document.body.contains(target)) {
+        throw new OwlError("Cannot mount a component on a detached dom node");
+      }
+      return;
+    }
   }
+  throw new OwlError("Cannot mount component: the target is not a valid DOM element");
 }
 
 export class EventBus extends EventTarget {

tests/app/__snapshots__/app.test.ts.snap

@@ -29,6 +29,19 @@ exports[`app can configure an app with props 1`] = `
 }"
 `;
 
+exports[`app can mount app in an iframe 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div class=\\"my-div\\"/>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    return block1();
+  }
+}"
+`;
+
 exports[`app destroy remove the widget from the DOM 1`] = `
 "function anonymous(app, bdom, helpers
 ) {

tests/app/app.test.ts

@@ -76,4 +76,22 @@ describe("app", () => {
       "Component 'Root' does not have a static props description"
     );
   });
+
+  test("can mount app in an iframe", async () => {
+    class SomeComponent extends Component {
+      static template = xml`<div class="my-div"/>`;
+    }
+
+    const iframe = document.createElement("iframe");
+    fixture.appendChild(iframe);
+    const app = new App(SomeComponent);
+    const iframeDoc = iframe.contentDocument!;
+    const comp = await app.mount(iframeDoc.body);
+    const div = iframeDoc.querySelector(".my-div");
+    expect(div).not.toBe(null);
+    expect(iframeDoc.contains(div)).toBe(true);
+    app.destroy();
+    expect(iframeDoc.contains(div)).toBe(false);
+    expect(status(comp)).toBe("destroyed");
+  });
 });