diff --git a/lib/editor/layout/page_viewport.dart b/lib/editor/layout/page_viewport.dart
new file mode 100644
index 0000000..d8c0c28
--- /dev/null
+++ b/lib/editor/layout/page_viewport.dart
@@ -0,0 +1,153 @@
+// lib/editor/layout/page_viewport.dart
+//
+// Continuous-single layout geometry: the PURE windowing math that decides which
+// pages are mounted for a given scroll position (P0.5 step 10). Pages stack
+// vertically; only the pages intersecting the viewport ± a cache band are
+// mounted (windowed lazy hosting → 60fps on a 300-page doc, R1).
+//
+// This file is intentionally widget-free and pdfrx-free: the actual page
+// mounting (a re-laid-out PdfPageView + the AnnotationLayer per page) and the
+// zoom-settle DPI refresh are device-gated and live in the viewport WIDGET +
+// pdf/page_tile.dart. The geometry is separated so it is unit-testable without
+// a GPU or a real document.
+
+import 'dart:math' as math;
+
+/// An inclusive range of page indices to mount. [isEmpty] when nothing
+/// intersects (e.g. an empty document or a scroll position past the end with no
+/// cache band reaching back).
+class PageWindow {
+  const PageWindow(this.first, this.last);
+
+  /// Sentinel empty window.
+  static const PageWindow empty = PageWindow(0, -1);
+
+  final int first;
+  final int last;
+
+  bool get isEmpty => last < first;
+  int get count => isEmpty ? 0 : (last - first + 1);
+
+  bool contains(int index) => !isEmpty && index >= first && index <= last;
+
+  @override
+  bool operator ==(Object other) =>
+      other is PageWindow && other.first == first && other.last == last;
+
+  @override
+  int get hashCode => Object.hash(first, last);
+
+  @override
+  String toString() => isEmpty ? 'PageWindow.empty' : 'PageWindow($first..$last)';
+}
+
+/// Vertical stacking metrics for continuous-single layout.
+///
+/// Page `i` occupies the half-open band `[offsetOf(i), offsetOf(i) + heightOf(i))`
+/// in content (scale-1) coordinates, with [gap] inserted between consecutive
+/// pages. Cumulative tops are precomputed so [visibleRange] is O(log n) per
+/// scroll frame.
+class PageStackMetrics {
+  PageStackMetrics({required List<double> pageHeights, this.gap = 0.0})
+      : assert(gap >= 0),
+        _heights = List<double>.unmodifiable(pageHeights),
+        _tops = _cumulativeTops(pageHeights, gap);
+
+  final List<double> _heights;
+
+  /// Top edge of each page in content coordinates (length == pageCount).
+  final List<double> _tops;
+
+  /// Gap between consecutive pages in content units.
+  final double gap;
+
+  int get pageCount => _heights.length;
+
+  /// Total scrollable content height (0 when there are no pages).
+  double get totalExtent {
+    if (_heights.isEmpty) return 0;
+    return _tops.last + _heights.last;
+  }
+
+  double heightOf(int index) => _heights[index];
+
+  /// Top edge (content coordinate) of page [index].
+  double offsetOf(int index) => _tops[index];
+
+  static List<double> _cumulativeTops(List<double> heights, double gap) {
+    final tops = List<double>.filled(heights.length, 0);
+    var acc = 0.0;
+    for (var i = 0; i < heights.length; i++) {
+      tops[i] = acc;
+      acc += heights[i] + gap;
+    }
+    return List<double>.unmodifiable(tops);
+  }
+
+  /// The inclusive page range intersecting the viewport
+  /// `[scrollOffset, scrollOffset + viewportExtent)` grown by [cacheExtent] on
+  /// each side. Pages whose band overlaps the grown window (even partially) are
+  /// included; the result is clamped to `[0, pageCount-1]`.
+  ///
+  /// Returns [PageWindow.empty] for an empty document or a window that does not
+  /// reach any page.
+  PageWindow visibleRange(
+    double scrollOffset,
+    double viewportExtent, {
+    double cacheExtent = 0.0,
+  }) {
+    if (_heights.isEmpty) return PageWindow.empty;
+    assert(viewportExtent >= 0);
+    assert(cacheExtent >= 0);
+
+    final double windowTop = scrollOffset - cacheExtent;
+    final double windowBottom = scrollOffset + viewportExtent + cacheExtent;
+
+    // No overlap with the content at all.
+    if (windowBottom <= 0 || windowTop >= totalExtent) {
+      return PageWindow.empty;
+    }
+
+    final int first = _firstIntersecting(windowTop);
+    final int last = _lastIntersecting(windowBottom);
+    if (last < first) return PageWindow.empty;
+    return PageWindow(first, last);
+  }
+
+  /// Lowest index whose band bottom is strictly after [y] (i.e. the first page
+  /// that the window's top edge does not sit fully below).
+  int _firstIntersecting(double y) {
+    // Find the first page whose bottom edge (top + height) > y.
+    var lo = 0;
+    var hi = _heights.length - 1;
+    var result = _heights.length - 1;
+    while (lo <= hi) {
+      final mid = (lo + hi) >> 1;
+      final bottom = _tops[mid] + _heights[mid];
+      if (bottom > y) {
+        result = mid;
+        hi = mid - 1;
+      } else {
+        lo = mid + 1;
+      }
+    }
+    return math.max(0, result);
+  }
+
+  /// Highest index whose top edge is strictly before [y].
+  int _lastIntersecting(double y) {
+    var lo = 0;
+    var hi = _heights.length - 1;
+    var result = 0;
+    while (lo <= hi) {
+      final mid = (lo + hi) >> 1;
+      if (_tops[mid] < y) {
+        result = mid;
+        lo = mid + 1;
+      } else {
+        hi = mid - 1;
+      }
+    }
+    return math.min(_heights.length - 1, result);
+  }
+}
diff --git a/test/page_viewport_test.dart b/test/page_viewport_test.dart
new file mode 100644
index 0000000..1027572
--- /dev/null
+++ b/test/page_viewport_test.dart
@@ -0,0 +1,107 @@
+// Unit tests for continuous-single windowing math (P0.5 step 10, automatable
+// slice). Pure geometry — no widgets, no pdfrx.
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/layout/page_viewport.dart';
+
+void main() {
+  group('PageStackMetrics geometry', () {
+    test('cumulative tops and total extent (uniform pages + gap)', () {
+      final m = PageStackMetrics(pageHeights: [100, 100, 100], gap: 10);
+      expect(m.pageCount, 3);
+      expect(m.offsetOf(0), 0);
+      expect(m.offsetOf(1), 110); // 100 + 10 gap
+      expect(m.offsetOf(2), 220);
+      expect(m.totalExtent, 320); // 220 + last height 100, no trailing gap
+    });
+
+    test('variable page heights', () {
+      final m = PageStackMetrics(pageHeights: [50, 200, 75]);
+      expect(m.offsetOf(0), 0);
+      expect(m.offsetOf(1), 50);
+      expect(m.offsetOf(2), 250);
+      expect(m.totalExtent, 325);
+    });
+
+    test('empty document', () {
+      final m = PageStackMetrics(pageHeights: []);
+      expect(m.pageCount, 0);
+      expect(m.totalExtent, 0);
+      expect(m.visibleRange(0, 800), PageWindow.empty);
+    });
+  });
+
+  group('PageStackMetrics.visibleRange', () {
+    final m = PageStackMetrics(pageHeights: List.filled(10, 100)); // 0..1000
+
+    test('top of document shows the first pages', () {
+      // viewport [0,250) → pages 0,1,2 (page 2 is [200,300), overlaps top 250)
+      expect(m.visibleRange(0, 250), const PageWindow(0, 2));
+    });
+
+    test('mid scroll shows the intersecting band', () {
+      // viewport [420,720) → pages 4 [400,500) .. 7 [700,800)
+      expect(m.visibleRange(420, 300), const PageWindow(4, 7));
+    });
+
+    test('cacheExtent grows the window on both sides', () {
+      // base [420,720) → 4..7; +150 cache → [270,870) → pages 2..8
+      expect(
+        m.visibleRange(420, 300, cacheExtent: 150),
+        const PageWindow(2, 8),
+      );
+    });
+
+    test('exact page boundary is half-open (no spurious extra page)', () {
+      // viewport [0,200): page 0 [0,100), page 1 [100,200). Page 2 starts at
+      // 200 which is the exclusive bottom → NOT included.
+      expect(m.visibleRange(0, 200), const PageWindow(0, 1));
+    });
+
+    test('last page at the bottom', () {
+      expect(m.visibleRange(950, 50), const PageWindow(9, 9));
+    });
+
+    test('clamps to valid indices at the ends', () {
+      // Overscroll below 0 (cache reaches negative) still starts at page 0.
+      expect(m.visibleRange(0, 100, cacheExtent: 500).first, 0);
+      // Overscroll past the end still ends at the last page.
+      expect(m.visibleRange(900, 200, cacheExtent: 500).last, 9);
+    });
+
+    test('window entirely above content is empty', () {
+      // Scrolled far past the end with no cache reaching back.
+      expect(m.visibleRange(5000, 300), PageWindow.empty);
+    });
+
+    test('window entirely below content (negative, no overlap) is empty', () {
+      // viewport sitting above page 0 with bottom <= 0.
+      expect(m.visibleRange(-1000, 500), PageWindow.empty);
+    });
+
+    test('PageWindow helpers', () {
+      const w = PageWindow(2, 5);
+      expect(w.count, 4);
+      expect(w.contains(2), isTrue);
+      expect(w.contains(5), isTrue);
+      expect(w.contains(6), isFalse);
+      expect(PageWindow.empty.isEmpty, isTrue);
+      expect(PageWindow.empty.count, 0);
+    });
+  });
+
+  group('visibleRange with gaps', () {
+    test('gap bands are not page bands (a scroll inside a gap shows neighbors)',
+        () {
+      final m = PageStackMetrics(pageHeights: [100, 100, 100], gap: 20);
+      // page0 [0,100), gap [100,120), page1 [120,220), gap [220,240), page2 [240,340)
+      // viewport [105,115) sits inside the first gap → nearest pages 0 and 1
+      // both within a tiny window? window [105,115): page0 bottom 100 < 105 so
+      // page0 excluded; page1 top 120 > 115 so excluded → empty band in gap.
+      expect(m.visibleRange(105, 10), PageWindow.empty);
+      // A slightly taller viewport spanning the gap catches both.
+      expect(m.visibleRange(90, 40), const PageWindow(0, 1));
+    });
+  });
+}