Two users editing the same file online could silently clobber each other:
the editor's save did a bare PUT with no precondition, even though the master
already enforces optimistic concurrency (fileapi.go checkIfMatch → 412). Now
the editor sends a precondition and surfaces a conflict UI instead of
overwriting.
- util.js: saveFile(node, content, contentType, opts) sends `If-Match: <etag>`
(or `If-Unmodified-Since` fallback) unless opts.force; returns {etag} from
the PUT response (so save→edit→save adopts the new version and doesn't
false-conflict); throws ConflictError (.status===412) on a precondition
failure so callers branch cleanly. New saveCopy() parks a conflicting edit
as `<stem>-conflict-<ts>.<ext>` (collision-probed) without losing either side.
- preview.js: getContentWithVersion(node) → {buf, etag, lastModified} captured
from the content GET (the listing JSON carries no per-file etag); threaded
into the editor ctx and exported. getArrayBuffer left untouched.
- conflict.js (new): shared, callback-driven dialog — mine-vs-theirs diff
(reuses zddc.diff + css/history.css) + Overwrite / Reload-theirs /
Save-a-copy / Cancel. Never calls saveFile/showFilePreview itself, so the
deferred Phase 5 cache-outbox conflict UI can reuse it with its own callbacks.
- preview-markdown.js / preview-yaml.js: capture + forward the version token,
adopt the returned etag on success, and on 412 open the dialog (Overwrite
re-fetches the current etag then re-saves — re-conflicts on a third writer
rather than blind-forcing; Reload clears dirty first so the renderInline
guard skips its confirm). FS-Access mode sends no precondition (no
concurrency) and never conflicts.
- build.sh: concat conflict.js after util.js.
- tests/conflict.spec.js (+ playwright project): If-Match sent, ConflictError
on 412, new-etag returned, force omits the precondition, dialog renders the
diff and each action resolves via its callback. Drives the fresh dist build
over file:// with a stubbed fetch (the test binary embeds the committed
browse.html, not dist, so a server-mode E2E would run stale code).
All browse + diff + conflict specs pass (18).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
