Closes#2070. The Map<rootId, Set<nodeId>> in useCanvasViewport.ts
accumulated entries indefinitely — adds on every successful auto-fit,
never deletes when a root left state.nodes (cascade delete or manual
remove). Operationally invisible until thousands of imports, but the
fix is cheap.
Adds pruneStaleSubtreeIds(map, liveNodeIds) — a pure helper exported
alongside the existing shouldFitGrowing helper, called at the top of
runFit before any read or write to the map. Bounds the map to "roots
present right now" instead of "every root ever auto-fitted in this
session." O(map_size) per fit; runs only at user-driven cadence.
Tests in __tests__/useCanvasViewport.test.ts cover the four cases:
delete-some / no-op / clear-all / never-add.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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