fix(core): stale computed values after a batch reverts a signal - #947
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Rolling the version back during batch snapshot reconciliation let a later write re-mint a version number that a lazy computed had already observed mid-batch, permanently serving a stale cached value. Fast-forward subscriber nodes that saw the pre-batch version instead.
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marvinhagemeister
approved these changes
Jul 5, 2026
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Root cause
reconcileBatchSnapshotsrolls a signal's_versionbackwards when a batch ends with the value equal to its pre-batch state. Version numbers are the only invalidation mechanism, and they must be monotonic: a lazy (unsubscribed) computed that reads the signal during the batch records the intermediate version on its dependency node. After the rollback, the next distinct write re-mints that exact version number for a different value, soneedsToRecomputesees matching versions and the computed serves its stale cache — permanently, on every read, even though the signal holds a new value:Reading a modified signal inside a batch is explicitly supported per
batch's documentation, and this shipped in@preact/signals-core@1.14.0.Fix
Never roll versions back. Instead, when a batch reverts a value, fast-forward the subscriber nodes that last saw the pre-batch version to the current version. Subscribed effects/computeds still skip recomputing on no-op batches (the point of the original optimization), while any node that observed an intermediate version keeps its mismatch and recomputes correctly. Lazy computeds that saw only the pre-batch version now do one value-equal recompute instead of being skipped — trading a micro-optimization for correctness.
Note: this PR was authored by Claude and @JoviDeCroock.