fix: release last_propagated_event after event propagation settles (#18569)

Fixes #18568

`last_propagated_event` was added in #16527 to stop Firefox from garbage collecting the event wrapper mid-propagation. The problem with this is that the even is now retained until the next event, which could be a while. This removes it after a macrotask.

Co-authored-by: rmurphy <rmurphy@fortressinfosec.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

.changeset/spotty-ducks-refuse.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: release `last_propagated_event` after event propagation settles so it no longer retains the last event's target subtree

packages/svelte/src/internal/client/dom/elements/events.js

@@ -159,12 +159,15 @@ export function delegate(events) {
 }
 
 // used to store the reference to the currently propagated event
-// to prevent garbage collection between microtasks in Firefox
+// to prevent garbage collection between microtasks in Firefox (<= 141)
 // If the event object is GCed too early, the expando __root property
 // set on the event object is lost, causing the event delegation
 // to process the event twice
 let last_propagated_event = null;
 
+// whether a task is already queued to clear `last_propagated_event`
+let last_propagated_event_clear_scheduled = false;
+
 /**
  * @this {EventTarget}
  * @param {Event} event
@@ -179,6 +182,21 @@ export function handle_event_propagation(event) {
 
 	last_propagated_event = event;
 
+	// The reference is only needed while the event can still reach another
+	// delegated root, i.e. during the current (synchronous) dispatch and its
+	// microtask checkpoints. Clearing it in a later task preserves the
+	// Firefox workaround while making sure the slot doesn't retain the last
+	// event forever — through `event.target` it would otherwise keep the
+	// entire detached subtree of whatever the user last clicked in alive
+	// until the next delegated event happens to arrive.
+	if (!last_propagated_event_clear_scheduled) {
+		last_propagated_event_clear_scheduled = true;
+		setTimeout(() => {
+			last_propagated_event_clear_scheduled = false;
+			last_propagated_event = null;
+		});
+	}
+
 	// composedPath contains list of nodes the event has propagated through.
 	// We check `event_symbol` to skip all nodes below it in case this is a
 	// parent of the `event_symbol` node, which indicates that there's nested