bug: diagram replacement retains stale walkthrough selection and fit work

[Wattenberger]

Two reproduced lifecycle defects in DiagramRenderer.svelte, tracked together because their local UI ownership repair is shared.

1. Replace diagram A with diagram B on the same mounted renderer. With disjoint state IDs, the old selected ID remains and B can render unfiltered paint. With overlapping IDs, the old local selection wins over B’s saved selection. Removing the locally selected state or changing stateless/stateful structure similarly leaves stale selection. A paused camera transition can remain held after identity replacement.
2. Toggle fit and unmount before the queued animation frame. The fit callback is not cancelled; repeated toggles enqueue multiple scroll writes rather than retaining only the latest frame.

Regression evidence on the preserved mixed frontend worktree at dd8276fe: canonical layered fixtures produced 8 failing unit cases and 1 passing selection-echo guard; two same-instance Chromium cases on an exact-source-matched preview failed at stale selected state (not readiness). A renderer-only candidate now passes the initial 9 unit cases; expanded lifecycle/continuity/browser/static gates are still pending.

Repair constraints: preserve valid local selections on equivalent objects and parent onUpdate echoes; never mutate incoming canonical payloads or emit unsolicited persistence updates; cancel obsolete transition paint and fit work. No routing, typography, frame sizing, footer redesign, or data normalization.

Tests: src/lib/components/diagrams/tests/diagram-lifecycle.test.ts and test/diagram-lifecycle.spec.ts. Local candidate is uncommitted; no PR or shipping claim.

Dedup searched diagram lifecycle/state/fit/replacement, walkthrough stale, and fit requestAnimationFrame. Existing diagram readiness/size/routing reports do not cover these reproduced ownership defects.

[github-actions]

Automated triage

Type set: Bug — Reproduced lifecycle defects with failing unit and browser test cases indicate a genuine bug, not a request or question.

Priority set: Medium — Stale selection and duplicate fit writes are real rendering correctness bugs but do not crash the app or lose data.

Effort set: Medium — Fix is scoped to one component's lifecycle handling but spans selection reconciliation and frame cancellation with multiple test suites still pending.

No additional labels suggested.

_{Automated agentic triage (pass 2). Labels, Type, and Priority/Effort fields are suggestions — maintainers may adjust them.}

[Wattenberger]

Local candidate validation completed on the preserved mixed worktree; not committed, merged, or shipped.

Final revision: 12/12 focused lifecycle units; 649/649 related tests; 19/19 declared-trigger tests; 104/104 UI invariants. Browser checks passed with one worker and zero retries: replacement/held-camera 2/2, shared-scene/camera continuity 4/4, local footer/fit/resize/focus 4/4, keyboard actions 1/1; the required verify:changed browser lane repeated replacement 2/2. Formatting, ESLint, architecture, renderer TypeScript and diff checks exited 0; Svelte reported 0 errors and 125 existing warnings.

Reconciliation handles identity replacement and removed local steps without unsolicited persistence updates, preserves valid local selections across parent echoes, and leaves first-mount prop seeding unchanged. Fit work is cancelled on repeated toggles, view reset, replacement and unmount. Replacement screenshots/geometry show unchanged 960x253 frame, 960x162 viewport and 960x45 local footer, with only B nodes remaining.

The two proven baseline failures and intermediate test-harness corrections are retained in local evidence. Native responsiveness and unrelated diagram work are not covered by this lifecycle validation. Issue stays open pending review and delivery.