Svelte 5 memory leak after transitions

[gyzerok]

Describe the bug

Transitions in Svelte 5 causes memory leaks by holding onto the DOM nodes indefinitely.

Also I found this PR which seems to be fixing a similar problem back in 2022 and this issue, that reports something similar for version 4.

In our case we are writing highly interactive real-time SPA, which means that user will go between screens a lot and will keep tab open for days, which gets amount of detached nodes to hundreds of thousands or maybe even millions. I can get from 1k to 20k nodes in a minute.

Reproduction

Here is the repo with the smallest example: https://github.com/gyzerok/svelte-transition-memory-leak.

Clone the repo and npm i && npm run dev.

Steps:

1. Open app in Chrome
2. Record memory snapshot
3. Click button twice
4. Record second memory snapshot
5. Compare both snapshots and filter by "detached"

Logs

No response

System Info

Chrome 127.0.6533.89

Severity

blocking all usage of svelte

[trueadm]

Are you sure this is related to transitions alone? If you remove the transitions do you see any memory leaks?

[gyzerok]

@trueadm yeah, if I remove the transition the leak is still there. Apparently it's really the {#key} tag that is causing the leak. Perhaps Svelte remembers previous keys even if they aren't needed anymore.

Remove the key and no leaks happen. And since key is a recommended way to start transitions in certain cases they just go hand in hand.

[trueadm]

@gyzerok Transitions still leak separately, but I also believe the key block is leaking here too. I have a PR for the transitions leaking, I'll put another up for key blocks.

[gyzerok]

@trueadm oh, that sounds amazing! Was a bit worried about those leaks and how much attention the issue would get. Really appreciate you working out on them!

[trueadm]

@gyzerok I'm unsure if the key block is leaking here, at least for me I can't see it is. If you remove the transition, then having the key block there or not doesn't seem to make any difference to me. Maybe you can show what you see in screenshots or with a video?

[trueadm]

FWIW you will always have a detached element because we use template cloning and retain the template for later so we can quickly clone it. However, this won't grow past that of what is needed for a template. This is intended and this is not a memory leak.

[gyzerok]

@trueadm here is the screenshot how it looks without transition added. It seems like you are right and if I click button multiple times then the amount of detached nodes does not grow (see diff between 2 and 1 is the same as between 4 and 1). So indeed this does not seem like a leak problem.

Anyway, eagerly awaiting your transition fix then. Thank you!

[gyzerok]

Hello @trueadm!

I've noticed that your fix got merged under 215 version. I've updated my reproduce repo with this version, but can still see the leak. After several button clicks I see 4 detached div nodes.

[trueadm]

Hello @trueadm!

I've noticed that your fix got merged under 215 version. I've updated my reproduce repo with this version, but can still see the leak. After 4 button clickes I see 4 detached div nodes.

If you press it again, do you see more than 4 detached nodes? 4 detached nodes might just be templates here.

[gyzerok]

If you press it again, do you see more than 4 detached nodes? 4 detached nodes might just be templates here.

Yeah, amount of leaking nodes corresponds to the amount of toggles for me. Also in the Object graph in the middle you can see that the retainer is an animation.