[FIX] concurrency: do not render delayed fibers when cancelled

Previously, if a fiber was delayed because one of its ancestors was
rendering, and that fiber was not a root fiber, it would be rendered
after all its ancestors had finished rendering even if one of those
ancestor renderings cancelled it.

This commit fixes that by simply checking that a delayed fiber is still
its component node's current fiber before rendering it.

src/component/fibers.ts

@@ -42,6 +42,10 @@ export function makeRootFiber(node: ComponentNode): Fiber {
   return fiber;
 }
 
+function throwOnRender() {
+  throw new Error("Attempted to render cancelled fiber");
+}
+
 /**
  * @returns number of not-yet rendered fibers cancelled
  */
@@ -49,6 +53,7 @@ function cancelFibers(fibers: Fiber[]): number {
   let result = 0;
   for (let fiber of fibers) {
     let node = fiber.node;
+    fiber.render = throwOnRender;
     if (node.status === STATUS.NEW) {
       node.destroy();
     }

src/component/scheduler.ts

@@ -32,7 +32,7 @@ export class Scheduler {
       let renders = this.delayedRenders;
       this.delayedRenders = [];
       for (let f of renders) {
-        if (f.root && f.node.status !== STATUS.DESTROYED) {
+        if (f.root && f.node.status !== STATUS.DESTROYED && f.node.fiber === f) {
           f.render();
         }
       }