The hierarchical 3-column drill-down was hard to scan and forced a stateful
walk per tag. Replaced with a flat, columns-as-tactics matrix that mirrors
attack.mitre.org/# — every cell is a one-click select target, with inline
sub-technique expand via a `+N` chevron.
- New endpoint GET /api/v1/mitre/matrix returns the full grid (tactics →
techniques → sub-techniques nested) in a single ~55 KB response, so the
SPA renders the whole matrix without firing 15 parallel queries. Two
pytest tests added (nested structure + auth required).
- MitreTagPicker.tsx rewritten as a horizontal-scrolling matrix:
- Click a tactic header → select the tactic (cyan filled).
- Click a technique cell → select the technique (orange filled).
- Click the `+N` chevron → expand sub-techniques inline within the column.
- Click a sub-technique → select (purple filled).
- Single Filter field matches on external_id or name across all kinds.
- Selection chips at the top, clickable to remove.
- `aria-pressed` on every clickable cell for screen readers and Playwright.
- e2e test updated to walk the new flow (click cell → assert aria-pressed,
expand chevron, click sub, verify chip + JSON preview, filter to T1078).
- Spec §F2 + §F12 + todo.md M4 entry updated to make the matrix layout the
canonical UI for MITRE tagging (so future spec-reviewer passes accept it).
- testing-m4.md walkthrough rewritten for the flat picker.
DoD post-refactor: make test-api → 53 passed (was 51), make e2e → 34 passed.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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