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When #traverse_effect_tree runs, it can execute block effects, which in turn can create effects which are scheduled, which means queued_root_effects is now filled again. We didn't take that into account and assumed that in #commit we would have no queued root effects. As a result another batch could wrongfully run with the queued root effects of the main batch. That in turn can mean that skipped_effects is different on the other batch, leading to some branches not getting traversed into. As a result part of the tree is marked clean while further below batches are still not marked clean. That then breaks reactivity as soon as you schedule an effect in this still-not-clean sub tree, as it will not bubble all the way up to the root, since it comes across a not-clean branch, assuming something was already scheduled.

The fix is simple: Stash the queued root effects before rebasing branches.

Fixes #17118

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When `#traverse_effect_tree` runs, it can execute block effects, which in turn can create effects which are scheduled, which means `queued_root_effects` is now filled again. We didn't take that into account and assumed that in `#commit` we would have no queued root effects. As a result another batch could wrongfully run with the queued root effects of the main batch. That in turn can mean that `skipped_effects` is different on the other batch, leading to some branches not getting traversed into. As a result part of the tree is marked clean while further below batches are still not marked clean. That then breaks reactivity as soon as you schedule an effect in this still-not-clean sub tree, as it will not bubble all the way up to the root, since it comes across a not-clean branch, assuming something was already scheduled.

The fix is simple: Stash the queued root effects before rebasing branches.

Fixes #17118
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changeset-bot bot commented Nov 12, 2025

🦋 Changeset detected

Latest commit: e3fa620

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package
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pnpm add https://pkg.pr.new/svelte@17145

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Experimental Async with preload and conditional bind:this kills reactivity

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