fix reactive updates not reflected when handle promise (#3660)

src/runtime/internal/await_block.ts

@@ -14,6 +14,8 @@ export function handle_promise(promise, info) {
 		const child_ctx = assign(assign({}, info.ctx), info.resolved);
 		const block = type && (info.current = type)(child_ctx);
 
+		let needs_flush = false;
+
 		if (info.block) {
 			if (info.blocks) {
 				info.blocks.forEach((block, i) => {
@@ -33,11 +35,15 @@ export function handle_promise(promise, info) {
 			transition_in(block, 1);
 			block.m(info.mount(), info.anchor);
 
-			flush();
+			needs_flush = true;
 		}
 
 		info.block = block;
 		if (info.blocks) info.blocks[index] = block;
+
+		if (needs_flush) {
+			flush();
+		}
 	}
 
 	if (is_promise(promise)) {

test/runtime/samples/await-set-simultaneous-reactive/_config.js

@@ -0,0 +1,13 @@
+export default {
+	html: `<p>wait for it...</p>`,
+	test({ assert, component, target }) {
+
+		return component.promise
+			.then(() => {
+				assert.htmlEqual(target.innerHTML, `
+					<p>the answer is 42!</p>
+					<p>the answer100 is 4200!</p>
+				`);
+			});
+	}
+};

test/runtime/samples/await-set-simultaneous-reactive/main.svelte

@@ -0,0 +1,21 @@
+<script>
+	let answer = 0;
+	$: answer100 = answer * 100;
+	export let promise = new Promise(resolve => {
+		setTimeout(() => {
+			resolve();
+			answer = 42;
+		}, 0)
+	});
+</script>
+
+{#if promise}
+	{#await promise}
+		<p>wait for it...</p>
+	{:then _}
+		<p>the answer is {answer}!</p>
+		<p>the answer100 is {answer100}!</p>
+	{:catch error}
+		<p>well that's odd</p>
+	{/await}
+{/if}