[FIX] component: destroy not yet mounted components (2)

Following 2922cee6ea

Previous commit was not correct: used children with a cancelled
fiber (for instance, with a new currentFiber) could be destroyed.
This commit fixes the issue differently, by directly detecting
children that are not used anymore (and not mounted), and destroy
them directly (no need to wait for willStart promise to be resolved
anymore).

src/component/component.ts

@@ -60,6 +60,9 @@ interface Internal<T extends Env, Props> {
   cmap: { [key: number]: number };
 
   currentFiber: Fiber | null;
+  // parentLastFiberId is there to help the parent component to detect, among
+  // its children, those that are not used anymore and thus can be destroyed
+  parentLastFiberId: number;
 
   boundHandlers: { [key: number]: any };
   observer: Observer | null;
@@ -169,6 +172,7 @@ export class Component<T extends Env, Props extends {}> {
       children: {},
       cmap: {},
       currentFiber: null,
+      parentLastFiberId: 0,
       boundHandlers: {},
       mountedCB: null,
       willUnmountCB: null,
@@ -579,6 +583,16 @@ export class Component<T extends Env, Props extends {}> {
         handlers: __owl__.boundHandlers,
         fiber: fiber
       });
+      // we iterate over the children to detect those that no longer belong to the
+      // current rendering: those ones, if not mounted yet, can (and have to) be
+      // destroyed right now, because they are not in the DOM, and thus we won't
+      // be notified later on (when patching), that they are removed from the DOM
+      for (let childKey in __owl__.children) {
+        let child = __owl__.children[childKey];
+        if (!child.__owl__.isMounted && child.__owl__.parentLastFiberId < fiber.id) {
+          child.destroy();
+        }
+      }
       if (!vnode) {
         throw new Error(`Rendering '${this.constructor.name}' did not return anything`);
       }