# A4 smart guides (`a4/guides.js`)

Pure functions, no React/DOM. `buildGuideCandidates(elements, excludeIds, pageSetup)`
→ `{V, H}`; `resolveSnap(draftRect, candidates, zoom)` → `{x, y, guides}`;
plus `unionRect(boxes)` and `snapToleranceMm(zoom)`.

## The threshold is in SCREEN PIXELS, not mm

`SNAP_PX = 6`, converted per-call via `SNAP_PX / (PX_PER_MM * zoom)`. This is
load-bearing, not a detail: a fixed mm threshold feels sticky zoomed in and
useless zoomed out. Do not "simplify" it to a constant mm value. The property
worth asserting is `tolMm * PX_PER_MM * zoom === SNAP_PX` at every zoom.

## Integration seams in `A4Canvas.jsx`

- Candidates are built **once** in `handleElementDragStart` into a `useRef`.
  Rebuilding per mousemove is the one thing that makes dragging laggy.
- Multi-select snaps the **union rect**, then applies one delta to every member.
  Snapping members independently tears the group apart.
- `hooks/useElementDrag.jsx` is shared with the matrix editor and forwards only
  pixel deltas — **never widen it**. Alt-to-suppress is tracked with a
  window-level `keydown`/`keyup` listener in A4Canvas, plus a `blur` reset (an
  alt-tab otherwise leaves snapping permanently off, because the keyup lands in
  another window).
- Bands emit no candidates (their edges *are* the margins, already candidates),
  but band children do.

## Gotcha: snapping breaks drag tests that assume raw deltas

Any pre-existing test that drags an element and asserts the exact px→mm result
can fail once a candidate happens to sit within ~1.6mm of where it lands. This
bit `a4Elements.test.js`'s rect case: it drifted 0.05mm from the 20mm left
margin and legitimately snapped. The fix is to hold Alt around the drag
(`withAltHeld` helper in that file), not to weaken the assertion — the test is
about unit conversion, so switching the magnet off states its intent.