diff --git a/packages/pagination/src/layout/__tests__/compose.spec.ts b/packages/pagination/src/layout/__tests__/compose.spec.ts
index 374b283477..f9ca250113 100644
--- a/packages/pagination/src/layout/__tests__/compose.spec.ts
+++ b/packages/pagination/src/layout/__tests__/compose.spec.ts
@@ -29,6 +29,10 @@ function snap(...blocks: MeasuredBlock[]): MeasuredSnapshot {
 }
 
 describe('composeLayout (place-whole / option C)', () => {
+  beforeEach(() => {
+    nextId = 0;
+  });
+
   it('places blocks that fit on a single page, stacked by height', () => {
     const out = composeLayout(snap(block(100), block(100), block(100)), INPUT);
     expect(out.pages).toHaveLength(1);
diff --git a/packages/pagination/src/layout/compose.ts b/packages/pagination/src/layout/compose.ts
index 4446bbfacb..f6be97837e 100644
--- a/packages/pagination/src/layout/compose.ts
+++ b/packages/pagination/src/layout/compose.ts
@@ -78,6 +78,9 @@ export function composeLayout(
     push({
       blockId: b.id,
       fragmentIndex: 0,
+      // Store the flow height so y-coordinate accounting is consistent with
+      // the real DOM flow (margins included). lineCount stays text-only for
+      // accurate line-level mapping (widows/orphans, caret projection).
       heightPx: flow,
       lineCount: b.lineCount,
       lineStart: 0,
diff --git a/packages/pagination/src/layout/continuous.ts b/packages/pagination/src/layout/continuous.ts
index 65ae503831..e425803ed1 100644
--- a/packages/pagination/src/layout/continuous.ts
+++ b/packages/pagination/src/layout/continuous.ts
@@ -19,19 +19,23 @@ function pageHeight(page: LayoutOutput['pages'][number]): number {
 }
 
 /**
- * The continuous-flow Y (px, relative to content top) at each interior page
- * boundary — i.e. the cumulative content height at the end of every page except
- * the last. N pages produce N-1 break Ys; the document end is not a break.
+ * The cumulative flow Y (px, relative to content top) at each interior page
+ * boundary — i.e. the sum of `fragment.heightPx` values for all pages up to
+ * (but not including) the last page. N pages produce N-1 break Ys; the
+ * document end is not a break.
  *
- * This is the pure pretext-flow position. The continuous overlay does not
- * position against it — DOM box spacing (margins) makes pretext's text-only
- * cumulative Y drift from the real rendered boundary. Prefer
- * {@link getContinuousBreaks}, which names the boundary block so the overlay can
- * anchor to that block's live DOM top.
+ * Note: `fragment.heightPx` stores the block's *flow* height (text height +
+ * DOM box spacing when `flowHeightPx` was supplied by the measurer). So these
+ * Y values include any per-block margin/padding/border accumulated across
+ * earlier pages. They are closer to the real DOM boundary than pure
+ * pretext-text heights, but still drift because adjacent-block margin
+ * collapse is not modelled. Prefer {@link getContinuousBreaks}, which names
+ * the boundary block so the overlay can anchor to that block's live DOM top —
+ * the only way to land exactly on a real block edge.
  */
 export function getContinuousBreakYs(layout: LayoutOutput): number[] {
   const breakYs: number[] = [];
-  let cumulative = 0;
+
 
   for (let i = 0; i < layout.pages.length - 1; i++) {
     cumulative += pageHeight(layout.pages[i]);