5.43.4+ $derived doesn't update

[abdel-17]

Describe the bug

I have a tree whose items can be expanded and deleted. For some strange reason, deleting an item causes the tree to stop updating after an item is expanded or collapsed. This bug doesn't happen in 5.43.3.

Reproduction

https://svelte.dev/playground/22fafc05ec84446fa2e4e4df0203449d?version=5.45.9

1. delete main.ts
2. try to expand Pictures

Logs

System Info

System:
    OS: macOS 26.1
    CPU: (10) arm64 Apple M4
    Memory: 154.14 MB / 16.00 GB
    Shell: 4.2.1 - /opt/homebrew/bin/fish
  Binaries:
    Node: 25.2.1 - /opt/homebrew/bin/node
    Yarn: 1.22.22 - /opt/homebrew/bin/yarn
    npm: 11.6.2 - /opt/homebrew/bin/npm
    pnpm: 10.24.0 - /opt/homebrew/bin/pnpm
    bun: 1.3.3 - /opt/homebrew/bin/bun
  Browsers:
    Brave Browser: 143.1.85.111
    Firefox: 145.0.2
    Safari: 26.1

Severity

annoyance

[yayza]

from https://svelte.dev/docs/svelte/$effect#Understanding-dependencies

$effect automatically picks up any reactive values ($state, $derived, $props) that are synchronously read inside its function body (including indirectly, via function calls) and registers them as dependencies. When those dependencies change, the $effect schedules a re-run.

If $state and $derived are used directly inside the $effect (for example, during creation of a reactive class), those values will not be treated as dependencies.

So

const expanded = $derived(expanded_ids.has(node.id));
const visible = $derived(parent === undefined || parent.expanded && parent.visible);

are never tracked and instead should be moved to the getters:

get expanded() {
  return expanded_ids.has(node.id);  // direct reads are tracked when accessed
},
get visible() {
  return parent === undefined || (parent.expanded && parent.visible);
},

why it worked before? i dunno 🤷

[abdel-17]

@yayza neither expanded nor visible are synchronously read inside an effect

[abdel-17]

Also, directly using a getter has very different performance characteristics. Particularly, the visible getter needs to be cached because, otherwise, it will recursively call itself up to the root.

[jdgamble555]

I am having same issue, but seems to be random with certain state signals. Something is broken recently with signals not firing when they should. Will try and find exact error.

[hmnd]

@abdel-17 using $derived inside $derived.by is the equivalent of using $derived in an $effect. It may have worked in 5.43.3 by coincidence, but the $derived is not doing anything here.

You can verify this for yourself by changing your playground to a version where it did work and adding a console.log in get visible().

I get identical results with or without $derived around the value of const visible:

If you're looking to truly make this efficient, you'd be better off simplifying to just ignore children when their parent is not visible. Something like this: https://svelte.dev/playground/2e417187e11c48d0b05ec3539e60d7ac?version=5.46.0

[abdel-17]

It doesn't matter if it's "technically an effect inside an effect." You should be able to create nested derived signals. It works in SolidJS.

https://playground.solidjs.com/anonymous/ebfbabf1-bd77-4838-9663-fd00288ba540