Add support for out-of-order render client side (#948)

React Server assumes that it can render in-order client side since the data
required to render each element will necessarily have arrived by the time it
is reached. In order to support moving the JS for non-critical above-the-fold
components into a secondary bundle, though, we would need support for
out-of-order rendering in the browser.

packages/react-server-test-pages/entrypoints.js

@@ -17,6 +17,10 @@ module.exports = {
 		entry: "/root/when",
 		description: "<RootElement when={...}>",
 	},
+	RootOrder: {
+		entry: "/root/order",
+		description: "Out of order render",
+	},
 	RootProvider: {
 		entry: "/root/rootProvider",
 		description: "<RootProvider store={...}>",

packages/react-server-test-pages/pages/root/order.js

@@ -0,0 +1,35 @@
+import {Component} from "react";
+import {RootElement} from "react-server";
+import Q from "q";
+
+class TurnGreen extends Component {
+	componentDidMount() {
+		this.setState({color: "green"});
+	}
+	render() {
+		const {color} = this.state || {};
+		return <div style={{backgroundColor: color || "red"}}>{this.props.children}</div>;
+	}
+}
+
+export default class RootOrderPage {
+	handleRoute(next) {
+		this.first = this.second = Q();
+
+		if (typeof window !== "undefined") {
+			this.first = Q.delay(1000);
+		}
+
+		return next();
+	}
+	getElements() {
+		return [
+			<RootElement when={this.first}>
+				<TurnGreen>This should turn green second</TurnGreen>
+			</RootElement>,
+			<RootElement when={this.second}>
+				<TurnGreen>This should turn green first</TurnGreen>
+			</RootElement>,
+		]
+	}
+}

packages/react-server/core/ClientController.js

@@ -604,12 +604,8 @@ class ClientController extends EventEmitter {
 		// As elements become ready, prime them to render as soon as
 		// their mount point is available.
 		//
-		// Always render in order to proritize content higher in the
-		// page.
-		//
-		elementPromisesOr.reduce((chain, promise, index) => chain
-			.then(() => promise
-				.then(element => rootNodePromises[index]
+		Q.all(elementPromisesOr.map((promise, index) => promise.then(
+				element => rootNodePromises[index]
 					.then(root => renderElement(element, root, index))
 					.catch(e => {
 						// The only case where this should evaluate to false is
@@ -619,9 +615,8 @@ class ClientController extends EventEmitter {
 							: 'element';
 						logger.error(`Error with element ${componentType}'s lifecycle methods at index ${index}`, e);
 					})
-				).catch(e => logger.error(`Error with element promise ${index}`, e))
-			),
-		Q()).then(retval.resolve);
+			).catch(e => logger.error(`Error with element promise ${index}`, e))
+		)).then(retval.resolve);
 
 		// Look out for a failsafe timeout from the server on our
 		// first render.