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

[NikolayMakhonin]

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:

1. The element is detached from the DOM but still referenced in last_propagated_event (an internal Svelte variable).
2. That element keeps its onfocus and onblur listeners.
3. 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 Graphics     
    Memory: 13.33 GB / 30.77 GB
  Binaries:
    Node: 20.19.1 - E:\Program Files\nodejs\node.EXE
    npm: 10.8.2 - E:\Program Files\nodejs\npm.CMD
    pnpm: 10.18.3 - C:\Users\Mika\AppData\Roaming\npm\pnpm.CMD
  Browsers:
    Chrome: 143.0.7499.109
    Edge: Chromium (125.0.2535.51)
    Firefox: 144.0.2 - E:\Program Files\Mozilla Firefox\firefox.exe
    Internet Explorer: 11.0.19041.3636
  npmPackages:
    rollup: 4.48.0 => 4.48.0 
    svelte: 5.46.0 => 5.46.0

Severity

annoyance