fix: transfer effects when merging batches#18254
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An effect could be gated behind a branch. If we don't defer + transfer them upon merge, the branch would still be marked clean but the effect behind it is dirty but no longer reachable. It's not reachable via mark either because that one only concerns itself with block/async effects, and the branch gating the effect is not guaranteed to be touched by that. Fixes #18249 Little sad side-effect: Since we cannot reliably know _before_ traversal if we have no blocking pending work left (the traversal could mark an if block falsy which contains the last blocker), we gotta undo a performance optimization.
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Rich-Harris
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May 20, 2026
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This PR was opened by the [Changesets release](https://github.com/changesets/action) GitHub action. When you're ready to do a release, you can merge this and the packages will be published to npm automatically. If you're not ready to do a release yet, that's fine, whenever you add more changesets to main, this PR will be updated. # Releases ## svelte@5.55.10 ### Patch Changes - fix: unlink errored and otherwise finished batch ([#18264](#18264)) - perf: walk composedPath() directly in delegated event propagation ([#18268](#18268)) - fix: transfer effects when merging batches ([#18254](#18254)) - fix: allow `$derived(await ...)` in disconnected effect roots ([#18273](#18273)) - fix: remove temporary raw-text hydration markers ([#18269](#18269)) - fix: propagate async `@const` blockers through closure references so template expressions like `{(() => host)()}` correctly wait for the awaited value ([#18309](#18309)) - fix: properly unlink batches ([#18298](#18298)) - fix: settle discarded batch ([#18290](#18290)) - fix: declare `let:` directives before `{@const}` declarations on slotted elements ([#18271](#18271)) - fix: resume outro-ed branches if they were kept around ([#18291](#18291)) - fix: avoid waterfall-warning when async resolves to same value ([#18297](#18297)) - fix: correctly coordinate component-level effects inside async blocks ([#18260](#18260)) - fix: make unnecessary commit work less likely ([#18263](#18263)) - chore: add tag name to `a11y_click_events_have_key_events` warning ([#18272](#18272)) - fix: catch rejected promises while merging/committing ([#18266](#18266)) Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
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An effect could be gated behind a branch. If we don't defer + transfer them upon merge, the branch would still be marked clean but the effect behind it is dirty but no longer reachable. It's not reachable via mark either because that one only concerns itself with block/async effects, and the branch gating the effect is not guaranteed to be touched by that.
Fixes #18249
Little sad side-effect: Since we cannot reliably know before traversal if we have no blocking pending work left (the traversal could mark an if block falsy which contains the last blocker), we gotta undo a performance optimization.