## Summary
A Staff Engineer pre-landing review (Greptile/CodeRabbit) on #1957
(merged) flagged four structural issues. This PR fixes the three that
were confirmed still present on `main`; the fourth (an unregistered-rule
`eslint-disable-next-line react-hooks/exhaustive-deps` comment) was
already fixed in #1957's second commit before merge and needed no
further change.
1. **`resolvePluginRuntime()` mislabeled "found but failed to init" as
`not_found`.** When a `runtimeHint` plugin registration is found but
`pluginContext`/`createRuntimeContext(...)` comes back falsy, the
resolver returned `reason: "not_found"` — indistinguishable from "never
registered" — defeating the point of a distinct `FallbackReason`. Now
returns `reason: "init_error"`. Updated the existing test that wrongly
asserted `"not_found"` for this path, and added a new test asserting all
three reachable `FallbackReason` values (`not_found`, `init_error`,
`factory_error`) are pairwise distinct.
2. **`ActiveAgentsPanel.tsx`/`AgentsView.tsx` hardcoded
`isInViewport={true}`.** Every agent card (live-agent header, board
card, list card) polled the runtime-fallback endpoint every 30s forever,
even scrolled off-screen — unlike `TaskCard.tsx`'s correct
`IntersectionObserver`-gated pattern. Both files now thread a real
`IntersectionObserver`-backed viewport signal into
`RuntimeFallbackBadge`. Added regression tests proving polling stops
once a badge instance's `isInViewport` transitions to `false` and
resumes once it goes back to `true` (desktop + a mobile-breakpoint
variant), plus verified via `tsc --noEmit` for `@fusion/dashboard`.
3. **Toast dedupe was per-hook-instance, not shared.**
`useRuntimeFallbackStatus`'s `lastToastedEventIdRef` was a local
`useRef`, so the same task rendered simultaneously in two card surfaces
(e.g. `ActiveAgentsPanel` + `AgentsView`) fired two separate toasts for
one fallback event. Dedupe now lives in module-level shared state (a
bounded `Map` keyed by `taskId:eventId`, FIFO-evicted past 500 entries)
so a fallback event toasts exactly once across every
simultaneously-mounted badge instance for the same task. Added a
cross-instance regression test mounting two badges for the same
`taskId`/`eventId` and asserting exactly one toast fires.
## Test evidence
- `pnpm --filter @fusion/engine exec vitest run
src/__tests__/runtime-resolution.test.ts --reporter=dot` — 25/25 pass
- `pnpm --filter @fusion/dashboard exec vitest run
app/components/__tests__/RuntimeFallbackBadge.test.tsx --reporter=dot` —
11/11 pass
- `pnpm --filter @fusion/dashboard run typecheck` — clean
- `pnpm --filter @fusion/engine run typecheck` — clean
## Scope
Isolated 6-file diff on top of current `main`
(`packages/engine/src/runtime-resolution.ts`,
`packages/engine/src/__tests__/runtime-resolution.test.ts`,
`packages/dashboard/app/hooks/useRuntimeFallbackStatus.ts`,
`packages/dashboard/app/components/ActiveAgentsPanel.tsx`,
`packages/dashboard/app/components/AgentsView.tsx`,
`packages/dashboard/app/components/__tests__/RuntimeFallbackBadge.test.tsx`).
No behavior outside the three findings above was touched.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
- **New Features**
- The desktop dashboard now supports plugin-backed runtime features,
improving how plugin-enabled workflows are loaded and run.
- Agent cards now pause background fallback polling when they’re
off-screen, helping the dashboard feel smoother and more responsive.
- **Bug Fixes**
- Improved runtime fallback handling so missing runtimes and
initialization failures are reported more accurately.
- Toast notifications are now better deduplicated, reducing repeated
alerts when multiple views show the same fallback state.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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