Known issues and gaps

An honest inventory of what is unfinished, what is knowingly imperfect, and what is out of scope. Updated whenever reality changes.

Code-level gaps

Legacy drag-and-drop files to delete. lib/dnd.ts, hooks/useOnDrag*.ts, hooks/index.ts and features/level/hooks/useDragState.ts are emptied placeholders (export {};) left over from the old HTML5 drag layer; the pointer DnD in features/level/dnd/ replaces them. They should be deleted outright.
Per-tile texture variation placeholders. components/Playground/textureVariant.ts and its test are emptied placeholders. The variation (rotated/mirrored/shifted tile textures) made fractional cell sizes look displaced and off-square, so it was removed (see The playground). The two files should be deleted.
Renderer length. SequenceItemRenderer.tsx still has one render branch per container block; further data-driving is optional cleanup, not a bug.

The interactive layers that need layout or timing are not covered. features/level/dnd/ (pointer geometry), the run animation in useSimulation, the router and the keyboard path. happy-dom has no layout engine, so a test there would assert on geometry that does not exist; the logic those pieces delegate to is tested directly instead. Components, providers, storage-backed state, the publish button, the toast stack, the playground and the skin provider are covered by the DOM harness (happydom.ts preload plus src/dom-harness.tsx).
A short list of lines stays dark, all redundant or defensive. They are re-checks that no input can reach: the simulator's flag short-circuits, the docs index comparator (docs.ts line 79), the switch tail in export-algorithm.ts, and the nested rebuild-failure propagation in stored-sequence.ts (lines 119/159/168). One check validator apiece in level.schema.ts and sequence.schema.ts is never reached either. They are left dark rather than exercised by synthetic tests.
Coverage is reported, not enforced. bunfig.toml turns coverage on but sets no coverageThreshold, so the gate is "all tests pass", not "every file is 100%".

Placing blocks by keyboard alone is not implemented (drag and drop has no keyboard alternative yet), and there are no screen-reader announcements during the run animation. Both are tracked on Accessibility.

The coverage table is the map of what is still dark. The remaining lines are redundant flag re-checks and a few defensive tails, listed on Testing; the real use cases are covered.
Bun.markdown is marked unstable upstream, and bun-types does not declare it yet; the app types the small surface it uses in features/documentation/markdown.ts and would only need that one file touched if the API changes.