[FIX] component: allow fragments in method mount

It is now possible to mount components in fragments

closes #494

doc/reference/component.md

@@ -239,7 +239,7 @@ We explain here all the public methods of the `Component` class.
 
 - **`mount(target)`** (async): this is the main way a
   component is added to the DOM: the root component is mounted to a target
-  HTMLElement. Obviously, this is asynchronous, since each children need to be
+  HTMLElement (or document fragment). Obviously, this is asynchronous, since each children need to be
   created as well. Most applications will need to call `mount` exactly once, on
   the root component.
 
@@ -247,6 +247,20 @@ We explain here all the public methods of the `Component` class.
   automatically re-rendered to ensure that changes in its state (or something
   in the environment, or in the store, or ...) will be taken into account.
 
+  If a component is mounted inside an element or a fragment which is not in the
+  DOM, then it will be rendered fully, but not active: the `mounted` hooks will
+  not be called. This is sometimes useful if we want to load an application in
+  memory. In that case, we need to mount the root component again in an element
+  which is in the DOM:
+
+  ```js
+  const app = new App();
+  await app.mount(document.createDocumentFragment());
+  // app is rendered in memory, but not active
+  await app.mount(document.body);
+  // app is now visible
+  ```
+
 - **`unmount()`**: in case a component needs to be detached/removed from the DOM, this
   method can be used. Most applications should not call `unmount`, this is more
   useful to the underlying component system.

src/component/component.ts

@@ -281,12 +281,12 @@ export class Component<T extends Env, Props extends {}> {
    *
    * Note that a component can be mounted an unmounted several times
    */
-  async mount(target: HTMLElement): Promise<void> {
+  async mount(target: HTMLElement | DocumentFragment): Promise<void> {
     const __owl__ = this.__owl__;
     if (__owl__.isMounted) {
       return Promise.resolve();
     }
-    if (!(target instanceof HTMLElement)) {
+    if (!(target instanceof HTMLElement || target instanceof DocumentFragment)) {
       let message = `Component '${this.constructor.name}' cannot be mounted: the target is not a valid DOM node.`;
       message += `\nMaybe the DOM is not ready yet? (in that case, you can use owl.utils.whenReady)`;
       throw new Error(message);

tests/component/component.test.ts

@@ -91,6 +91,17 @@ describe("basic widget properties", () => {
     expect(fixture.innerHTML).toBe("<div>content</div>");
   });
 
+  test("can be mounted on a documentFragment", async () => {
+    class SomeWidget extends Component<any, any> {
+      static template = xml`<div>content</div>`;
+    }
+    const widget = new SomeWidget();
+    await widget.mount(document.createDocumentFragment());
+    expect(fixture.innerHTML).toBe("");
+    await widget.mount(fixture);
+    expect(fixture.innerHTML).toBe("<div>content</div>");
+  });
+
   test("display a nice message if mounted on a non existing node", async () => {
     class SomeWidget extends Component<any, any> {
       static template = xml`<div>content</div>`;