diff --git a/src/ui/diff/plannedReviewRows.ts b/src/ui/diff/plannedReviewRows.ts
index 9bc09f7..5d8a985 100644
--- a/src/ui/diff/plannedReviewRows.ts
+++ b/src/ui/diff/plannedReviewRows.ts
@@ -1,5 +1,6 @@
 import type { LayoutMode } from "../../core/types";
 import { measureAgentInlineNoteHeight } from "../components/panes/AgentInlineNote";
+import type { SectionGeometry, VerticalBounds } from "../lib/diffSpatial";
 import { reviewRowId } from "../lib/ids";
 import type { PlannedReviewRow } from "./reviewRenderPlan";
 
@@ -15,19 +16,13 @@ export interface PlannedReviewRowLayoutOptions {
  *
  * The row ids let DiffPane upgrade from planned measurements to exact mounted measurements later.
  */
-export interface PlannedHunkBounds {
-  top: number;
-  height: number;
+export interface PlannedHunkBounds extends VerticalBounds {
   startRowId: string;
   endRowId: string;
 }
 
 /** Aggregate geometry for one file section measured from planned review rows. */
-export interface PlannedSectionGeometry {
-  bodyHeight: number;
-  hunkAnchorRows: Map<number, number>;
-  hunkBounds: Map<number, PlannedHunkBounds>;
-}
+export type PlannedSectionGeometry = SectionGeometry<PlannedHunkBounds>;
 
 /** Return whether this planned row should count toward a hunk's own visible extent. */
 function rowContributesToHunkBounds(row: PlannedReviewRow) {
diff --git a/src/ui/lib/diffSectionGeometry.ts b/src/ui/lib/diffSectionGeometry.ts
index e46a4bd..17b7109 100644
--- a/src/ui/lib/diffSectionGeometry.ts
+++ b/src/ui/lib/diffSectionGeometry.ts
@@ -4,21 +4,17 @@ import { buildSplitRows, buildStackRows } from "../diff/pierre";
 import { measureRenderedRowHeight, findMaxLineNumber } from "../diff/renderRows";
 import type { PlannedHunkBounds } from "../diff/plannedReviewRows";
 import { buildReviewRenderPlan, type PlannedReviewRow } from "../diff/reviewRenderPlan";
+import type { SectionGeometry, VerticalBounds } from "./diffSpatial";
 import { reviewRowId } from "./ids";
 import type { VisibleAgentNote } from "./agentAnnotations";
 import type { AppTheme } from "../themes";
 
-export interface DiffSectionRowBounds {
+export interface DiffSectionRowBounds extends VerticalBounds {
   key: string;
-  top: number;
-  height: number;
 }
 
 /** Cached placeholder sizing and hunk navigation geometry for one file section. */
-export interface DiffSectionGeometry {
-  bodyHeight: number;
-  hunkAnchorRows: Map<number, number>;
-  hunkBounds: Map<number, PlannedHunkBounds>;
+export interface DiffSectionGeometry extends SectionGeometry<PlannedHunkBounds> {
   rowBounds: DiffSectionRowBounds[];
   rowBoundsByKey: Map<string, DiffSectionRowBounds>;
 }
diff --git a/src/ui/lib/diffSpatial.ts b/src/ui/lib/diffSpatial.ts
new file mode 100644
index 0000000..8320487
--- /dev/null
+++ b/src/ui/lib/diffSpatial.ts
@@ -0,0 +1,17 @@
+/** One vertical extent measured in terminal rows within a single coordinate space. */
+export interface VerticalBounds {
+  top: number;
+  height: number;
+}
+
+/**
+ * Shared geometry for one file section body.
+ *
+ * `bodyHeight` and every nested `top` value should use the same coordinate space, such as
+ * section-body-relative rows or whole-stream rows.
+ */
+export interface SectionGeometry<THunkBounds extends VerticalBounds> {
+  bodyHeight: number;
+  hunkAnchorRows: Map<number, number>;
+  hunkBounds: Map<number, THunkBounds>;
+}