[zxch3n]

Summary

doc.checkout(doc.frontiers()) detached a loro.js document even when those frontiers were the latest version, so later imports stopped reaching its state. loro-crdt leaves the document attached. A loro.js peer and a Rust peer that ran the same calls then diverged, and the differential fuzz saw it as lost imports.

doc.checkout(doc.frontiers());
doc.import(remoteUpdate);
doc.getText("t").toString();  // main: old text; loro-crdt: includes the update

Rationale

Rust's _checkout_without_emitting_with_event returns early when the target equals the state frontiers, re-attaching when they are the latest frontiers. It also returns early (keeping the attach state) when the shrunk target equals them, e.g. a target with a dominated extra frontier. loro.js now does the same, clears #checkoutVersion when it re-attaches, and emits no event, like Rust. Checking out the latest version from an older one still leaves the document detached; only checkoutToLatest()/attach() re-attach from there, as in Rust.

The existing test "records imports in the oplog while the state is detached" relied on the old behavior to detach; it now calls detach().

Validation

• New test in loro-js/tests/runtime.test.ts covering: attached checkout of the current version (imports still apply), a dominated extra frontier, a repeated checkout of an old version, old → latest staying detached, and detach() then checkout(frontiers) re-attaching and accepting local edits. Fails on main. Every assertion was checked against loro-crdt (WASM build of this main), which returns the same attach state in all cases.
• pnpm --dir loro-js format:check, lint, typecheck, test (248 passed), build.
• bench:complexity (1k–8k), 7 alternating rounds vs main, loadavg 25–34: 284 items, none both >1.5× and >0.3 ms slower. Total 1301 ms on main, 1258 ms here.

Found by the differential fuzz harness (#1141), and listed by #1132 as a non-MovableList divergence.

🤖 Generated with Claude Code

[github-actions]

WASM Size Report

• Original size: 3277.59 KB
• Gzipped size: 1089.33 KB
• Brotli size: 762.83 KB

[zxch3n]

Merged the latest main (c0ff1255). The only conflict was two helpers appended at the end of document.ts; both are kept. Main's checkoutToLatest attach step resets the same two fields (#checkoutVersion, #detached) as this early return, so the merged logic is consistent.

Still needed on main: doc.checkout(doc.frontiers()) detaches, so a later import does not reach the state (xy instead of xyB). loro-crdt and this branch give xyB and match loro-crdt on all 8 attach-state probes. pnpm -C loro-js check with LORO_REQUIRE_WASM_REFERENCE=1 passes (25 files, 415 tests).