chore: avoid microtasks when flushing sync (alternative) (#15895)

* chore: avoid microtasks when flushing sync

* chore: avoid microtasks when flushing sync

* chore: avoid microtasks when flushing sync

* tweak

* WIP

* another

* more

* tweak

* fix

* fix

* belt and braces

* Revert "belt and braces"

This reverts commit f8de1cf47a.

---------

Co-authored-by: Dominic Gannaway <trueadm@users.noreply.github.com>

.changeset/polite-melons-tickle.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: avoid microtasks when flushing sync

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

@@ -823,6 +823,8 @@ export function flushSync(fn) {
 	if (fn) {
 		is_flushing = true;
 		flush_queued_root_effects();
+
+		is_flushing = true;
 		result = fn();
 	}