Handle TerrainExtension offset layers in getDepthLayers

modules/core/src/lib/deck-picker.ts

@@ -1027,7 +1027,10 @@ export default class DeckPicker {
 
   /**
    * Determine which layers to use for the depth (pickZ) pass.
-   * - If a non-draped layer was picked, use just that layer.
+   * - If a non-terrain-dependent layer was picked, use just that layer.
+   * - If an offset layer was picked, include terrain layers alongside it so
+   *   TerrainEffect.preRender can bind the heightmap and the layer renders at
+   *   its correct elevation (terrain_z + data_z).
    * - If a draped layer was picked (geometry is at z=0) or no layer was picked
    *   (e.g. no-FBO tiles at extreme zoom), fall back to terrain layers.
    */
@@ -1036,12 +1039,17 @@ export default class DeckPicker {
       return [];
     }
     const {pickedLayer} = pickInfo;
-    const isDraped = pickedLayer?.state?.terrainDrawMode === 'drape';
-    if (pickedLayer && !isDraped) {
+    const terrainLayers = pickableLayers.filter(l => l.props.operation.includes('terrain'));
+    const terrainDrawMode = pickedLayer?.state?.terrainDrawMode;
+    if (pickedLayer && terrainDrawMode === 'offset') {
+      // For offset layers, include terrain layers alongside
+      return [pickedLayer, ...terrainLayers];
+    }
+    if (pickedLayer && terrainDrawMode !== 'drape') {
       return [pickedLayer];
     }
     // For draped layers or when no layer was picked, use terrain layers for depth
-    return pickableLayers.filter(l => l.props.operation.includes('terrain'));
+    return terrainLayers;
   }
 
   /**

test/modules/core/lib/deck-picker.spec.ts

@@ -147,3 +147,41 @@ test('DeckPicker#pick async empty', async () => {
   layerManager.finalize();
   deckPicker.finalize();
 });
+
+test('DeckPicker#getDepthLayers', () => {
+  const deckPicker = new DeckPicker(device);
+  // depthFBO must exist or the method early-returns []
+  deckPicker.depthFBO = {destroy: () => {}};
+
+  const terrainLayer = {props: {operation: 'terrain+draw'}, state: {}};
+  const offsetLayer = {props: {operation: 'draw'}, state: {terrainDrawMode: 'offset'}};
+  const drapedLayer = {props: {operation: 'draw'}, state: {terrainDrawMode: 'drape'}};
+  const regularLayer = {props: {operation: 'draw'}, state: {}};
+
+  const pickableLayers = [offsetLayer, terrainLayer, regularLayer, drapedLayer];
+  const noPick = {pickedColor: null, pickedObjectIndex: -1};
+
+  // unproject3D disabled → always []
+  expect(deckPicker._getDepthLayers(noPick, pickableLayers, false)).toEqual([]);
+
+  // nothing picked → terrain layers
+  expect(deckPicker._getDepthLayers(noPick, pickableLayers, true)).toEqual([terrainLayer]);
+
+  // regular layer picked → just that layer
+  expect(
+    deckPicker._getDepthLayers({...noPick, pickedLayer: regularLayer}, pickableLayers, true)
+  ).toEqual([regularLayer]);
+
+  // drape layer picked → terrain layers (geometry is at z=0, need terrain for depth)
+  expect(
+    deckPicker._getDepthLayers({...noPick, pickedLayer: drapedLayer}, pickableLayers, true)
+  ).toEqual([terrainLayer]);
+
+  // offset layer picked → [pickedLayer, ...terrainLayers] so TerrainEffect.preRender
+  // doesn't early-exit and the heightmap is available for the depth pass
+  expect(
+    deckPicker._getDepthLayers({...noPick, pickedLayer: offsetLayer}, pickableLayers, true)
+  ).toEqual([offsetLayer, terrainLayer]);
+
+  deckPicker.finalize();
+});