The reviewer identified two bugs in the original PR:
1. PATCH /api/config leaves session.dimensions stale: LoadConfig()
derives dimensions from the old dm_scope, and the merge carries
those stale dimensions forward. ApplyDmScope() then exits early
because dimensions is already populated, causing a mismatch between
dm_scope (new) and dimensions (old).
2. Legacy/default configs omit dm_scope in GET response: configs with
explicit dimensions but no dm_scope (including DefaultConfig) return
no dm_scope field, causing the frontend to fall back to its default
('per-channel-peer'), which may not match the actual dimensions.
Fix:
- Add DeriveDmScope() to reverse-map known dimensions arrays to
dm_scope when dm_scope is empty.
- Call it in LoadConfig(), PUT handler, PATCH handler, and
ResetToDefaults() for consistent normalization.
- In PATCH handler, clear stale dimensions from the merge result when
the patch contains session.dm_scope but not session.dimensions,
allowing ApplyDmScope() to re-derive from the new scope.
- Add comprehensive unit tests for DeriveDmScope() and scope
transition scenarios.
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