fix: ensure reactions are correctly attached for unowned deriveds

[trueadm]

Fixes #15149. We weren't setting the active_reaction when processing effects and checking if they were dirty, which mean the skip_reaction logic was not treating unowned deriveds as connected even though they should as we're flushing effects at this stage of reconciliation.

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pnpm add https://pkg.pr.new/svelte@15158

[dummdidumm]

could you explain what deriveds were unowned in this example? Looking at the generated code, shouldn't it be seen as owned (directly created in an effect; the root effect)?

[trueadm]

could you explain what deriveds were unowned in this example? Looking at the generated code, shouldn't it be seen as owned (directly created in an effect; the root effect)?

Deriveds are no longer attached to effect. Effects and deriveds no longer own deriveds too – this was changed in a previous PR because it was leading to bugs where the derived was being destroyed when the effect or owning derived re-runs. If you create a derived in something like a class and then pass that out of the owning derived or effect then it makes no sense for it suddenly get destroyed by something else.

Deriveds are marked as unowned when they're read outside of a tracked context – in this case from the props.js logic that reads it for the $bindable() logic. We mark them as unowned as nothing owns them and we don't want them to possible leak.

[dummdidumm]

Ok, so asking differently - in which cases are deriveds still owned? Because it sounds like they are almost never owned now?

[trueadm]

@dummdidumm A derived being unowned doesn't mean it still can't be connected to the graph – they are mostly connected to the graph when a tracking effect references them. If a derived is only ever read inside an effect, it is never set as unowned too.