make everything non-breaking for people who dont opt in

packages/svelte/src/internal/client/dom/operations.js

@@ -5,6 +5,7 @@ import { init_array_prototype_warnings } from '../dev/equality.js';
 import { get_descriptor, is_extensible } from '../../shared/utils.js';
 import { active_effect } from '../runtime.js';
 import { EFFECT_RAN } from '../constants.js';
+import { async_mode_flag } from '../../flags/index.js';
 
 // export these for reference in the compiled code, making global name deduplication unnecessary
 /** @type {Window} */
@@ -214,6 +215,8 @@ export function clear_text_content(node) {
  * current `<svelte:boundary>`
  */
 export function should_defer_append() {
+	if (!async_mode_flag) return false;
+
 	var flags = /** @type {Effect} */ (active_effect).f;
 	return (flags & EFFECT_RAN) !== 0;
 }

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

@@ -45,7 +45,7 @@ import {
 } from './reactivity/deriveds.js';
 import * as e from './errors.js';
 import { FILENAME } from '../../constants.js';
-import { tracing_mode_flag } from '../flags/index.js';
+import { async_mode_flag, tracing_mode_flag } from '../flags/index.js';
 import { tracing_expressions, get_stack } from './dev/tracing.js';
 import {
 	component_context,
@@ -823,7 +823,19 @@ export function process_effects(batch, root) {
 			} else if (is_branch) {
 				effect.f ^= CLEAN;
 			} else if ((flags & RENDER_EFFECT) !== 0) {
-				batch.render_effects.push(effect);
+				// we need to branch here because in legacy mode we run render effects
+				// before running block effects
+				if (async_mode_flag) {
+					batch.render_effects.push(effect);
+				} else {
+					try {
+						if (check_dirtiness(effect)) {
+							update_effect(effect);
+						}
+					} catch (error) {
+						handle_error(error, effect, null, effect.ctx);
+					}
+				}
 			} else if ((flags & EFFECT) !== 0) {
 				batch.effects.push(effect);
 			}

packages/svelte/tests/runtime-legacy/samples/lifecycle-render-beforeUpdate/_config.js

@@ -2,12 +2,6 @@ import { test } from '../../test';
 import { flushSync } from 'svelte';
 
 export default test({
-	// this test breaks because of the changes required to make async work
-	// (namely, running blocks before other render effects including
-	// beforeUpdate and $effect.pre). Not sure if there's a good
-	// solution. We may be forced to release 6.0
-	skip: true,
-
 	async test({ assert, target, logs }) {
 		const input = /** @type {HTMLInputElement} */ (target.querySelector('input'));
 		assert.equal(input?.value, 'rich');