Add page navigation to PDF previews

Acceptance Criteria

• A loaded PDF displays an outlined page navigator immediately left of the zoom pill, with a stacked previous/next stepper and a current / total counter.
• Manual scrolling reports the rendered page nearest the viewport center.
• When two pages are equally near the viewport center, the lower-numbered page remains current.
• Previous and next smoothly scroll the adjacent page so its top is at the viewport top.
• Previous is disabled on page 1 and next is disabled on the final page; either action is also disabled until its target page anchor is available.
• A one-page document displays 1 / 1 with both navigation actions disabled.
• The navigator is absent until the PDF page count is known and remains absent if PDF rendering fails.
• Navigation preserves continuous scrolling, text selection, zoom behavior, and pane-local control visibility.
• Icon controls have accessible names, visible focus, keyboard operation, and legible disabled states in light and dark themes; the current page status is announced politely when it changes.

Testing Decisions

Extend the existing Preview browser regression suite with a multi-page PDF fixture. Cover initial page status, a geometry assertion that the navigator precedes zoom without scroll-rail collision, adjacent-page navigation, manual-scroll updates, boundary and unavailable-target disabled states, and the one-page state. Retain the current PDF selection and zoom regressions as compatibility coverage.

Key Decisions

• Continuous scroll remains the viewer model; this feature navigates already-rendered pages rather than introducing a single-page viewer.
• The PDF renderer owns page-anchor discovery and active-page observation. The Preview control host displays that state and requests a page movement.
• The control uses the current Preview auto-hide lifecycle and existing tokenized control and icon conventions.

Constraints & Invariants

• No global, workspace, or tab-level page state is introduced.
• The page status never renders before a page count is known.
• A missing page anchor disables the corresponding navigation action rather than advancing the status optimistically.
• Page navigation does not replace the existing PDF error state.

Prior Art

• Existing PDF canvas renderer and Preview control group.
• Existing Preview zoom, text-selection, and scroll-rail regression contracts.

Source

Brainstorming session, 2026-09-10.

Notes

Single standard-slice UI feature. No ADR or sub-issue decomposition is warranted.