add test, tidy up

packages/svelte/src/internal/client/runtime.js

@@ -969,15 +969,6 @@ export function get(signal) {
 		captured_signals.add(signal);
 	}
 
-	if (DEV && from_async_derived) {
-		var tracking = (from_async_derived.f & REACTION_IS_UPDATING) !== 0;
-		var was_read = from_async_derived.deps !== null && from_async_derived.deps.includes(signal);
-
-		if (!tracking && !was_read) {
-			w.await_reactivity_loss();
-		}
-	}
-
 	// Register the dependency on the current reaction signal.
 	if (active_reaction !== null && !untracking) {
 		if (derived_sources !== null && derived_sources.includes(signal)) {
@@ -1043,8 +1034,17 @@ export function get(signal) {
 		}
 	}
 
+	if (DEV) {
+		if (from_async_derived) {
+			var tracking = (from_async_derived.f & REACTION_IS_UPDATING) !== 0;
+			var was_read = from_async_derived.deps !== null && from_async_derived.deps.includes(signal);
+
+			if (!tracking && !was_read) {
+				w.await_reactivity_loss();
+			}
+		}
+
 		if (
-		DEV &&
 			tracing_mode_flag &&
 			tracing_expressions !== null &&
 			active_reaction !== null &&
@@ -1064,6 +1064,7 @@ export function get(signal) {
 				entry.read.push(get_stack('TracedAt'));
 			}
 		}
+	}
 
 	return signal.v;
 }

packages/svelte/tests/runtime-runes/samples/async-reactivity-loss/_config.js

@@ -0,0 +1,26 @@
+import { flushSync, tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+
+	html: `<button>a</button><button>b</button><p>pending</p>`,
+
+	async test({ assert, target, warnings }) {
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await tick();
+		flushSync();
+		assert.htmlEqual(target.innerHTML, '<button>a</button><button>b</button><h1>3</h1>');
+
+		assert.deepEqual(warnings, ['Detected reactivity loss']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-reactivity-loss/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	let a = $state(1);
+	let b = $state(2);
+
+	async function a_plus_b() {
+		return await a + await b;
+	}
+</script>
+
+<button onclick={() => a++}>a</button>
+<button onclick={() => b++}>b</button>
+
+<svelte:boundary>
+	<h1>{await a_plus_b()}</h1>
+
+	{#snippet pending()}
+		<p>pending</p>
+	{/snippet}
+</svelte:boundary>