Memory leak in onfocus/onblur/etc listeners and last_propagated_event (Svelte 5 / Svelte 4)

Why this is a good first issue

Memory leak due to unsubscribed event listeners in Svelte's internal handling.

AI Summary

The issue describes a memory leak where certain event listeners (onfocus, onblur) are not properly unsubscribed when elements are detached from the DOM, potentially retaining large objects in memory. The fix would require modifying Svelte's internal event handling logic, but the exact scope across versions (Svelte 4 vs 5) needs verification. The reproduction case is clear but the solution might touch core event propagation logic.

Issue Description

Describe the bug

At least the onfocus and onblur event listeners are not being unsubscribed. Other listeners I haven’t checked. onclick listeners are properly cleaned up on Svelte 5, but not on Svelte 4

Here’s how the memory leak happens in my case:

The element is detached from the DOM but still referenced in last_propagated_event (an internal Svelte variable).
That element keeps its onfocus and onblur listeners.
Those listeners may hold references to any app objects — potentially large ones — causing unnecessary memory retention.

Reproduction

Svelte 5: https://svelte.dev/playground/127c05413cf1475998aa0fcefa2ab10f?version=5.46.0 Svelte 4: https://svelte.dev/playground/7fb7862b534a44009161bdf28de1d3a2?version=5.46.0

<script>
	let visible = $state(true);

	let elems = []
	if (typeof window !== 'undefined') {
	  window.elems = elems
	}

	let elem = $state(null);
	$effect(() => {
		if (elem != null) {
			elems.push(elem)
		}
		console.log(elem)
	})

	function onClick() {
		console.log('click')
	}
	function onFocus() {
		console.log('focus')
	}
	function onBlur() {
		console.log('blur')
	}
</script>

<button onclick={() => visible = !visible}>Show/Hide</button>

<br>
<br>

{#if visible}
	Test Element: 
  <input
		type="radio"
		bind:this={elem}
		onclick={onClick}
		onfocus={onFocus}
		onblur={onBlur}
	/>
{/if}

<div>
  <br>
	1) Click on the Show/Hide many times
  <br>
	2) Run this code in dev tools to see remaining events listeners:
	<br>
	<code>
		elems.map(o => getEventListeners(o))
	</code>
	<br>
	3) onclick was unsubscribed but onfocus/onblur stil there
	<br>
	* If an element becomes Detached but is still retained in memory for various reasons, this leads to retention of not only the listeners but also all objects held by these functions, which can lead to large memory leaks
</div>

Logs

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (16) x64 AMD Ryzen 7 PRO 6850U with Radeon Gr