Lead Time breakdown missing time when review events are absent

[hhaiyang]

Description:

The Lead Time for Changes breakdown loses time between ready-for-review and merge when review or approval events are missing from a PR.

The three middle components (First Response Time, Rework Time, Merge Time) depend on chained anchor events — first review and approval. When those events don't exist, all three resolve to zero and the elapsed time vanishes from the breakdown. This is confusing — the PR table shows a real lead time, but the breakdown suggests no time was spent in review, rework, or merge. Teams looking at the breakdown to understand where time goes get a misleading picture. As a consequence, the five breakdown components no longer sum to the total lead time.

The issue affects any PR where the review chain is incomplete:

Scenario	First Response Time	Rework Time	Merge Time	Time lost
Review + Approval	ready→review ✅	review→approval ✅	approval→merge ✅	None
Review, no Approval	ready→review ✅	0	0	review→merge
No Review, no Approval	0	0	0	ready→merge

Steps to reproduce:

1. Open a PR and mark it ready for review
2. Merge the PR without any reviews or approvals
3. Wait for the next sync cycle
4. Open the Lead Time breakdown in the team view
5. Compare the sum of the five breakdown components against the per-PR lead time in the PR table

Expected behavior:

The full elapsed time between ready-for-review and merge is accounted for in the breakdown. The five components (First Commit to Open, First Response Time, Rework Time, Merge Time, Merge to Deploy) sum to the total lead time.

For example, a PR that is ready for review at Mon 09:00 and merged at Wed 09:00 with no reviews should show 48h attributed to a sub-component.

Actual behavior:

First Response Time, Rework Time, and Merge Time are all zero. The 48h between ready-for-review and merge is missing from the breakdown entirely.

Example: First Commit to Open = 1h, First Response Time = 0, Rework Time = 0, Merge Time = 0, Merge to Deploy = 1h → Total = 2h (48h missing).

Server Setup Information:

• Deployment Method: docker (middlewareeng/middleware:latest)

Additional context

Suggested fix: Each component's end-anchor should fall through to the next available event in the chain, with merge as the final fallback:

Scenario	First Response Time	Rework Time	Merge Time
Review + Approval	ready→review	review→approval	approval→merge
Review, no Approval	ready→review	review→merge	0
No Review, no Approval	ready→merge	0	0

This requires no new components, data model changes, or UI changes. It's semantically accurate ("time waiting for first response" is exactly what happened when no review ever came) and reconciles the math for all scenarios.

Related: #207 (tracks surfacing unreviewed PRs as a feature). This issue is specifically about the lead time breakdown components not summing to the total when review events are absent.

[THEAbhishekjoshi]

@hhaiyang @dhruvagarwal
I investigated this issue and reproduced the missing-time behavior locally.

The root cause appears to be that the current implementation discards breakdown stages (first_review_time, rework_time, merge_time) whenever review/approval events are absent by setting them to -1/None.

As a result, portions of the elapsed PR lifecycle time are not attributed to any breakdown component, even though the total cycle time remains correct.