diff --git a/src/polygon/CesiumPolygonOverlayRenderer.ts b/src/polygon/CesiumPolygonOverlayRenderer.ts
index 7a01afd..8ef4446 100644
--- a/src/polygon/CesiumPolygonOverlayRenderer.ts
+++ b/src/polygon/CesiumPolygonOverlayRenderer.ts
@@ -1,5 +1,5 @@
 import { ArcType, Cartesian3, Entity, PolygonGraphics, PolygonHierarchy, PolylineGraphics } from 'cesium';
-import { WGS84Geodesic, Planar, resolveHoles, type GeoPoint, type PolygonAddParams, type PolygonChangeParams, type PolygonEntity, type PolygonOverlayRenderer, type PolygonState } from '@mapconductor/js-sdk-core';
+import { densifyPolygonRing, resolveHoles, type GeoPoint, type PolygonAddParams, type PolygonChangeParams, type PolygonEntity, type PolygonOverlayRenderer, type PolygonState } from '@mapconductor/js-sdk-core';
 import { CesiumMapViewHolder } from '../CesiumMapViewHolder';
 import { toCesiumColor } from '../color';
 import { pointsToDegrees } from '../helpers';
@@ -9,6 +9,8 @@ import { POLYGON_Z_BASE } from '../zOrder';
 // Same segment length react-for-arcgis uses for polygons: coarser than
 // polylines so world-scale rings don't densify into huge geometries.
 const GEODESIC_MAX_SEGMENT_LENGTH_METERS = 100_000;
+// Planar.createInterpolatePoints' default, the same as polylines use.
+const LINEAR_MAX_SEGMENT_LENGTH_METERS = 10_000;
 
 export class CesiumPolygonOverlayRenderer implements PolygonOverlayRenderer<Entity> {
   constructor(readonly holder: CesiumMapViewHolder) {}
@@ -27,10 +29,15 @@ export class CesiumPolygonOverlayRenderer implements PolygonOverlayRenderer<Enti
     // one. Densify each ring ourselves — great-circle interpolation when
     // geodesic, linear lon/lat interpolation (Google Maps' straight-line
     // semantics) otherwise — so the two modes produce their intended shapes.
+    // Each ring includes its closing edge (last vertex -> first) in both
+    // modes: interpolating only between neighbours leaves that edge one long
+    // rhumb segment in the fill while the outline curved along it.
     const densify = (points: GeoPoint[]): GeoPoint[] =>
-      state.geodesic
-        ? WGS84Geodesic.createInterpolatePoints(points, GEODESIC_MAX_SEGMENT_LENGTH_METERS)
-        : Planar.createInterpolatePoints(points);
+      densifyPolygonRing(
+        points,
+        state.geodesic,
+        state.geodesic ? GEODESIC_MAX_SEGMENT_LENGTH_METERS : LINEAR_MAX_SEGMENT_LENGTH_METERS,
+      );
 
     // No height: the polygon renders as ground geometry, which is what makes
     // Cesium honour zIndex ordering against other overlays (polylines sit in
@@ -61,7 +68,11 @@ export class CesiumPolygonOverlayRenderer implements PolygonOverlayRenderer<Enti
           width: state.strokeWidth,
           material: toCesiumColor(state.strokeColor),
           clampToGround: true,
-          arcType: ArcType.GEODESIC,
+          // Same arc type as the fill: between the densified vertices both
+          // fill and outline take the rhumb (lon/lat) segment, so they trace
+          // one line. With GEODESIC the outline bowed away from the fill on
+          // every segment, most visibly on long undensified ones.
+          arcType: ArcType.RHUMB,
           zIndex: POLYGON_Z_BASE + state.zIndex,
         })
       : undefined;
diff --git a/test/PolygonClosingEdge.test.mjs b/test/PolygonClosingEdge.test.mjs
new file mode 100644
index 0000000..c9f21c2
--- /dev/null
+++ b/test/PolygonClosingEdge.test.mjs
@@ -0,0 +1,75 @@
+import assert from 'node:assert/strict';
+import { mkdirSync, mkdtempSync, rmSync } from 'node:fs';
+import { after, test } from 'node:test';
+import { fileURLToPath, pathToFileURL } from 'node:url';
+import { resolve } from 'node:path';
+import { build } from 'tsup';
+import { createGeoPoint, createPolygonState } from '@mapconductor/js-sdk-core';
+
+// The fill and the outline of a polygon must trace the same line, closing edge
+// (last vertex -> first) included, in both geodesic modes. Cesium is replaced
+// by a stub that records the graphics the renderer builds.
+mkdirSync(new URL('../build/', import.meta.url), { recursive: true });
+const outDir = mkdtempSync(new URL('../build/polygon-closing-edge-tests-', import.meta.url));
+after(() => rmSync(outDir, { recursive: true, force: true }));
+const cesiumStub = fileURLToPath(new URL('./stubs/cesium.mjs', import.meta.url));
+await build({
+    entry: { renderer: 'src/polygon/CesiumPolygonOverlayRenderer.ts' },
+    outDir, format: ['esm'], platform: 'node', splitting: false,
+    dts: false, sourcemap: false, config: false, silent: true,
+    external: ['@mapconductor/js-sdk-core'],
+    noExternal: ['cesium'],
+    esbuildOptions(options) { options.alias = { cesium: cesiumStub }; },
+});
+const { CesiumPolygonOverlayRenderer } = await import(pathToFileURL(resolve(outDir, 'renderer.mjs')));
+
+const p = (latitude, longitude) => createGeoPoint({ latitude, longitude });
+const BOSTON = p(42.36, -71.06);
+const LONDON = p(51.5, -0.12);
+const ICELAND = p(64.1, -21.9);
+const TRIANGLE = [BOSTON, LONDON, ICELAND];
+
+async function render(geodesic) {
+    const entities = [];
+    const holder = {
+        isDestroyed: () => false,
+        map: { entities: { add: (entity) => (entities.push(entity), entity) }, scene: { requestRender() {} } },
+    };
+    const state = createPolygonState({
+        id: 'tri', points: TRIANGLE, geodesic, strokeWidth: 2, strokeColor: '#000', fillColor: '#f00',
+    });
+    await new CesiumPolygonOverlayRenderer(holder).onAdd([{ state }]);
+    return entities[0];
+}
+
+/** [lon, lat, lon, lat, ...] -> [[lon, lat], ...] */
+const pairs = (degrees) => degrees.reduce((acc, v, i) => (i % 2 ? acc[acc.length - 1].push(v) : acc.push([v]), acc), []);
+
+for (const geodesic of [true, false]) {
+    test(`geodesic=${geodesic}: fill and outline use the same vertices and arc type`, async () => {
+        const entity = await render(geodesic);
+        const fill = pairs(entity.polygon.hierarchy.positions);
+        const outline = pairs(entity.polyline.positions);
+        // The outline is the fill ring closed with its first vertex.
+        assert.deepEqual(outline.slice(0, -1), fill);
+        assert.deepEqual(outline[outline.length - 1], fill[0]);
+        assert.equal(entity.polyline.arcType, entity.polygon.arcType);
+    });
+}
+
+test('geodesic=false: the closing edge is densified along the lat/lng line', async () => {
+    const fill = pairs((await render(false)).polygon.hierarchy.positions);
+    // Halfway along Iceland -> Boston the linear edge passes 53.23N at 46.48W.
+    const midLng = (ICELAND.longitude + BOSTON.longitude) / 2;
+    const midLat = (ICELAND.latitude + BOSTON.latitude) / 2;
+    const nearest = Math.min(...fill.map(([lng, lat]) => Math.hypot(lat - midLat, lng - midLng)));
+    assert.ok(nearest < 0.1, `nearest ${nearest}`);
+});
+
+test('geodesic=true: the closing edge is densified along the great circle', async () => {
+    const fill = pairs((await render(true)).polygon.hierarchy.positions);
+    // Iceland -> Boston passes 55.6N at 53.2W (the undensified edge stays about 4 degrees away).
+    // Geodesic rings are densified every 100 km, so allow half a segment.
+    const nearest = Math.min(...fill.map(([lng, lat]) => Math.hypot(lat - 55.63, lng + 53.17)));
+    assert.ok(nearest < 1, `nearest ${nearest}`);
+});
diff --git a/test/stubs/cesium.mjs b/test/stubs/cesium.mjs
new file mode 100644
index 0000000..d843cc5
--- /dev/null
+++ b/test/stubs/cesium.mjs
@@ -0,0 +1,13 @@
+// Minimal stand-in for 'cesium' that records what the polygon renderer builds.
+export const ArcType = { NONE: 'NONE', GEODESIC: 'GEODESIC', RHUMB: 'RHUMB' };
+export class Color { static WHITE = new Color(); static fromCssColorString() { return new Color(); } }
+export const Cartesian3 = { fromDegreesArray: (degrees) => [...degrees] };
+export class Cartesian2 {}
+export class Cartographic {}
+export const SceneTransforms = {};
+const CesiumMath = {};
+export { CesiumMath as Math };
+export class Entity { constructor(options) { Object.assign(this, options); } }
+export class PolygonHierarchy { constructor(positions, holes = []) { this.positions = positions; this.holes = holes; } }
+export class PolygonGraphics { constructor(options) { Object.assign(this, options); } }
+export class PolylineGraphics { constructor(options) { Object.assign(this, options); } }