Manually `mount`ed component fails to update after DOM reinsertion

[SkylerLipthay]

Describe the bug

Component rendering breaks after manually inserting a mounted component into one DOM node and then moving it into another.

I'm not sure if this sort of thing was ever officially supported. I haven't found anything in the docs. This pattern worked in 5.22.2 but broke in 5.22.3 (5 months ago) due to effect processing changes.

My exact use case is a complex "tiling window manager" component akin to GoldenLayout. The basic requirement is to preserve the exact DOM state of a component instance when it's conditionally rendered in different locations. The component contains complex internal state (form inputs, scroll position, third-party widget state, etc.) that can't be serialized/restored. I need the exact same component instance DOM to be reused rather than destroyed/recreated.

I've written a minimal reproduction below. Pressing the inc button works as expected until the swap button is pressed, then the inc button stops working.

If this is an unsupported technique, any advice would be appreciated.

Reproduction

• https://svelte.dev/playground/6534c37164ee409d8c21fcb4ee58e91b?version=5.38.6 (fails)
• https://svelte.dev/playground/6534c37164ee409d8c21fcb4ee58e91b?version=5.22.3 (fails)
• https://svelte.dev/playground/6534c37164ee409d8c21fcb4ee58e91b?version=5.22.2 (works)

System Info

Binaries:
  Node: 24.6.0
  npm: 11.5.1
Browsers:
  Chromium: 138.0.7204.183
npmPackages:
  svelte: 5.38.6 => 5.38.6

Severity

blocking an upgrade, regression

[7nik]

When the component is mounted, it is attached to the parent effect tree. After swapping, it is detached (now two effect trees) but it gets marked as inert (not reactive). Probably it is a bug.

Workaround - prevent attaching to the parent tree on mounting:

---	onMount(() => {
+++	onMount(async () => {
+++		await Promise.resolve();
		const counter = cached();
		el.append(counter);
	});