PageStackMetrics + PageWindow: the pure geometry that decides which pages are
mounted for a scroll position (windowed lazy hosting → 60fps on a 300-page doc,
R1). Pages stack vertically with cumulative tops (O(log n) binary-search
visibleRange); a viewport [scroll, scroll+extent) grown by cacheExtent on each
side selects the inclusive intersecting page band, half-open at page boundaries,
clamped to valid indices, empty for empty/over-scrolled-past documents, and
gap-aware (a scroll resting inside an inter-page gap shows no page).
Widget-free + pdfrx-free by design: the page-mounting widget and zoom-settle DPI
refresh are device-gated; only the windowing math is automatable, and it is here
with exhaustive unit coverage (boundaries, cache band, clamping, gaps, empty).
flutter analyze lib/editor clean; 115/115 tests (+13).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
