WIP

Author: Rich-Harris

packages/svelte/src/internal/client/reactivity/batch.js

@@ -584,6 +584,9 @@ function infinite_loop_guard() {
 	}
 }
 
+/** @type {Effect[] | null} */
+export let eager_block_effects = null;
+
 /**
  * @param {Array<Effect>} effects
  * @returns {void}
@@ -600,6 +603,8 @@ function flush_queued_effects(effects) {
 		if ((effect.f & (DESTROYED | INERT)) === 0 && is_dirty(effect)) {
 			var sv = schedule_version;
 
+			eager_block_effects = [];
+
 			update_effect(effect);
 
 			// Effects with no dependencies or teardown do not get added to the effect tree.
@@ -619,14 +624,24 @@ function flush_queued_effects(effects) {
 				}
 			}
 
+			if (eager_block_effects.length > 0) {
+				for (const e of eager_block_effects) {
+					update_effect(e);
+				}
+
+				eager_block_effects = [];
+			}
+
 			// if a state change in a user effect invalidates a _different_ effect,
 			// abort and reschedule in case that effect now needs to be destroyed
 			if (schedule_version > sv && (effect.f & USER_EFFECT) !== 0) {
-				break;
+				// break;
 			}
 		}
 	}
 
+	eager_block_effects = null;
+
 	while (i < length) {
 		schedule_effect(effects[i++]);
 	}

packages/svelte/src/internal/client/reactivity/sources.js

@@ -33,7 +33,7 @@ import * as e from '../errors.js';
 import { legacy_mode_flag, tracing_mode_flag } from '../../flags/index.js';
 import { get_stack, tag_proxy } from '../dev/tracing.js';
 import { component_context, is_runes } from '../context.js';
-import { Batch, schedule_effect } from './batch.js';
+import { Batch, eager_block_effects, schedule_effect } from './batch.js';
 import { proxy } from '../proxy.js';
 import { execute_derived } from './deriveds.js';
 
@@ -340,7 +340,13 @@ function mark_reactions(signal, status) {
 		if ((flags & DERIVED) !== 0) {
 			mark_reactions(/** @type {Derived} */ (reaction), MAYBE_DIRTY);
 		} else if (not_dirty) {
-			if ((flags & BLOCK_EFFECT) !== 0) schedule_version += 1;
+			if ((flags & BLOCK_EFFECT) !== 0) {
+				if (eager_block_effects !== null) {
+					eager_block_effects.push(/** @type {Effect} */ (reaction));
+				}
+				schedule_version += 1;
+			}
+
 			schedule_effect(/** @type {Effect} */ (reaction));
 		}
 	}