Backport Vtk

Backporting upstream VTK fixes into cvista

cvista is frozen on VTK 9.6.2 (upstream tag v9.6.2, project vtk/vtk id 13 on gitlab.kitware.com). Upstream keeps moving — its master is on the 9.7.0 cycle. Real bug fixes and useful features land upstream after the 9.6.2 cut, and cvista should pull in the ones that affect the modules it ships. Each backport is one MR → one PR, linked to the MR.

cvista's root commit is a squash of 9.6.2, so there is no shared git history with upstream. git cherry-pick does not work. A backport applies the MR's diff and adapts it to the 9.6.2 tree by hand where context has drifted.

The reusable details (API calls, diff fetch, triage heuristics) are in references/upstream-api.md. The per-MR landing is delegated to the vtk-backport-engineer agent. The ledger of what has been backported, skipped, or is pending is docs/upstream-backports.md.

The sweep

1. Read the ledger docs/upstream-backports.md. The watermark is the highest MR iid (or the date) already triaged. If the ledger is absent, this is the first sweep: create it and use the 9.6.2 tag date as the floor.

2. Pull merged MRs since the watermark from the GitLab API (read-only, public). See references/upstream-api.md for the exact query. Collect iid, title, merge date, changed file paths, and web_url for each.

3. Triage each MR (heuristics in the reference file; the short version):

   • Skip if every changed file is absent from the cvista tree (git ls-files <path> empty) — that module was trimmed, so the fix is irrelevant. This is the strongest, cheapest filter.
   • Skip infra, docs, CI, ThirdParty bumps, deprecation churn, and changes only to modules cvista does not ship.
   • Take crash / correctness / memory-safety fixes touching shipped modules (highest value). Take user-visible feature or behavior fixes case by case.
   • Defer performance MRs by default — cvista has its own perf line under the parity contract, and an upstream perf change may conflict with it. Flag, do not auto-land.
   • When unsure, mark pending in the ledger and surface it for a human call rather than guessing.

4. Present the triage as a table (iid, title, decision, reason) and confirm scope before landing anything. Landing N PRs is a high-volume, outward-facing action; get a go-ahead.

Landing the accepted backports

For each accepted MR, spawn a vtk-backport-engineer agent (run them in parallel for independent MRs; use worktree isolation so concurrent branches do not collide). Give each agent:

• the MR iid, title, and web_url
• the changed-file list
• the standing instructions below

Each agent, for its one MR:

1. Branch backport/vtk-mr-<iid>-<short-slug> from main.
2. Fetch the MR diff (references/upstream-api.md) and apply it. Try git apply --3way first; where context has drifted on the 9.6.2 tree, port the change by hand, reading the MR to preserve its intent. Keep the change minimal and matched to surrounding VTK style.
3. Handle parity: a behavioral fix diverges from stock 9.6.2 on purpose. Update the affected tests/bitexact/ op to the corrected expectation and note the divergence (see the parity-contract skill). A crash/guard fix on degenerate input usually leaves valid-input output unchanged, so add a regression test for the degenerate case instead.
4. Build (PROFILE=fast ./build-cvista.sh) and run the relevant gate (bitexact / renderexact / regression) to prove the fix and the parity story.
5. Open the PR. The body must link the upstream MR (https://gitlab.kitware.com/vtk/vtk/-/merge_requests/<iid>), summarize the upstream defect in cvista's own words, state the parity bucket, and attach the gate evidence. Commit and PR prose follow .claude/CLAUDE.md (single-line conventional commit, no AI attribution, prose-hygiene pass). Keep it framed around PyVista and VTK; no downstream-consumer rationale.
6. Report back the PR number and the parity outcome.

Close the loop

Update docs/upstream-backports.md: for each MR, record iid, title, decision (backported #<PR> / skipped: <reason> / pending: <reason> / deferred: perf), and the date. Advance the watermark. The next sweep starts from there, so triage is incremental and never repeats work.

Ledger format

# Upstream VTK backports

Watermark: MR iid <N> (swept <YYYY-MM-DD>). Base: VTK 9.6.2 (gitlab.kitware.com/vtk/vtk).

| MR | Title | Decision | Date |
|----|-------|----------|------|
| !13353 | vtkPolyLine::Clip nullptr crash on degenerate polylines | backported #120 | 2026-06-21 |
| !13355 | Fix perf issues in marshalling | skipped: SerializationManager not shipped | 2026-06-21 |