# ADR 0007 — The pane-scroll room is the reading room

**Status:** accepted (2026-07-04) · **Decider:** Fritz · **Supersedes:** the page-scroll canvas presentation of [ADR 0005](/demarkus-library/adr/0005-reading-room-spatial-trail.md) (the trail model itself is unchanged)

## Context

Since R2 the trail canvas was one page-level scroller: panes laid left→right,
the page scrolling vertically as a unit. Reading deep into one document
dragged every neighbor with it; the margin column (ADR 0005 R1) reserved
~290px per focused pane for metadata that is consulted occasionally, not
continuously. The sliding-panes alternative was parked as "ADR territory"
until prototyped behind `DEMARKUS_PANE_SCROLL` (v0.16.0) and walked.

## Decision

The **pane-scroll room** becomes the default presentation of the trail canvas:

1. **Fixed-viewport room** — `100dvh` flex column (nav / canvas / dock);
   the page never scrolls vertically.
2. **Every pane is its own scroller** — reading depth is per-pane;
   neighbors, nav, and dock never move. Sticky paper-backed pane heads keep
   the address rail visible; interior column rules separate reading columns;
   scrollbars are thin and hover-only. Spines run floor-to-ceiling.
3. **The margin is summoned, not docked** — the catalog record (status,
   tags, OKF, provenance, hashes, backlinks, escape links) opens as the
   **metadata lens**: `?meta=<paneIndex>` URL state, the reader lens's record
   twin (pane-head "meta" affordance; scrim/✕/Esc/Back close; ordinary
   clicks dismiss). Doc panes only.
4. **Reading columns share the screen** — expanded panes `flex: 1 1 60ch`,
   bounded 46–82ch so measure survives; the freed margin width goes to text.
5. **At-rest trust signal** — the status chip rides the pane head (the
   room's one color signal), since the margin badge now waits behind the
   lens.
6. **The flag inverts** — `DEMARKUS_PANE_SCROLL` defaults true and is the
   opt-out back to the page-scroll room, retained until that room is retired.

Presentation only: same markup contract, same trail URLs, same no-JS
behavior (the lens is URL state; `<details>`/PRG idioms unchanged).

## Consequences

- **Sidenotes are hidden in the room** (their float needs the margin); they
  remain in the reader overlay, which keeps the full Tufte pane. If sidenote
  use grows, revisit (e.g. inline expansion).
- **Preview cards must escape pane clips** — solved by CSS anchor
  positioning (`position: fixed` + per-pair inline anchor names + flip
  fallbacks). Browsers without it (Firefox today) keep absolute upward cards
  that clip near a pane top — an accepted degrade until anchor positioning
  ships there.
- **Librarian transcript keeps its own bounded scroller** — it is what pins
  the view to the newest exchange while streaming and keeps the ask form in
  reach inside a scrolling pane.
- **Mobile is unresolved** — 46ch min columns on a phone mean horizontal
  panning; neither room was designed for small screens. Open follow-up.
- **Dock/margin rework is now unblocked**: the page-scroll room's margin CSS
  (grid columns, sidenote floats) stays for the reader overlay and the
  opt-out room; retire with the flag.
- Rejected: keeping the experiment opt-in indefinitely (two first-class
  rooms to maintain); JS-measured card positioning (anchor positioning does
  it declaratively).

## History

Prototype + verdict walk: [journal 2026-07-03-ux](/demarkus-library/journal/2026-07-03-ux.md).
Shipped as the default in the release after v0.16.0.