diff --git a/scripts/format-scope.json b/scripts/format-scope.json
index 4b5ae815f..a708c6b74 100644
--- a/scripts/format-scope.json
+++ b/scripts/format-scope.json
@@ -50,6 +50,7 @@
   "src/queries/heights.js",
   "src/render/cesium/internals.js",
   "src/render/cesium/cameraInput.js",
+  "src/render/cesium/ctrlWheelZoom.js",
   "src/render/cesium/cameraInput.test.mjs",
   "src/render/input/router.js",
   "src/render/input/router.test.mjs",
diff --git a/scripts/package-boundaries.json b/scripts/package-boundaries.json
index 8077a3ceb..9eb75f2a9 100644
--- a/scripts/package-boundaries.json
+++ b/scripts/package-boundaries.json
@@ -2448,6 +2448,7 @@
       "src/app/surfaceServices.js",
       "src/app/tools.js",
       "src/app/viewer.js",
+      "src/render/cesium/ctrlWheelZoom.js",
       "src/sky/starrySky.js",
       "src/sky/stars.js",
       "src/app/atmosphereCompat.js",
diff --git a/src/app/application.test.mjs b/src/app/application.test.mjs
index 3e9079b87..d756e967c 100644
--- a/src/app/application.test.mjs
+++ b/src/app/application.test.mjs
@@ -2,7 +2,7 @@ import test from 'node:test';
 import assert from 'node:assert/strict';
 import { spawnSync } from 'node:child_process';
 import { createApplication } from './application.js';
-import { installTrackpadPinchZoom } from './viewer.js';
+import { installTrackpadPinchZoom } from '../render/cesium/ctrlWheelZoom.js';
 
 const phases = ['Scene', 'Controls', 'Data', 'Tools'];
 function fixture(overrides = {}) {
diff --git a/src/app/scene.js b/src/app/scene.js
index 87f9736ee..b1405108b 100644
--- a/src/app/scene.js
+++ b/src/app/scene.js
@@ -1,6 +1,6 @@
 import { createApplicationOperations } from './operations.js';
 import * as Cesium from 'cesium';
-import { installTrackpadPinchZoom } from '../app/viewer.js';
+import { installTrackpadPinchZoom } from '../render/cesium/ctrlWheelZoom.js';
 import {
   cesiumRenderer,
   cesiumViewerOf,
diff --git a/src/app/viewer.js b/src/app/viewer.js
index c8b841352..9bc72b7f4 100644
--- a/src/app/viewer.js
+++ b/src/app/viewer.js
@@ -2,107 +2,6 @@ import * as Cesium from 'cesium';
 import { applyModelAtmosphereWorkaround } from './atmosphereCompat.js';
 import { StarrySky } from '../sky/starrySky.js';
 
-const PINCH_ZOOM_MULTIPLIER = 8;
-const MAX_PINCH_PIXEL_DELTA = 120;
-
-function boundedPinchDelta(delta) {
-  if (!Number.isFinite(delta) || delta === 0) return delta;
-  return (
-    Math.sign(delta) *
-    Math.min(Math.abs(delta) * PINCH_ZOOM_MULTIPLIER, MAX_PINCH_PIXEL_DELTA)
-  );
-}
-
-/**
- * Add browser trackpad pinch to Cesium's zoom inputs and return its disposer.
- * Browsers expose this gesture as a small pixel-mode Ctrl+wheel event.
- */
-export function installTrackpadPinchZoom(
-  viewer,
-  { createWheelEvent = (type, init) => new WheelEvent(type, init) } = {},
-) {
-  const controller = viewer?.scene?.screenSpaceCameraController;
-  const container = viewer?.container;
-  const canvas = viewer?.canvas;
-  if (!controller || !container || !canvas)
-    throw new TypeError('A complete Cesium viewer is required');
-
-  const originalZoomEventTypes = controller.zoomEventTypes;
-  const zoomEventTypes = Array.isArray(originalZoomEventTypes)
-    ? originalZoomEventTypes
-    : originalZoomEventTypes === undefined
-      ? []
-      : [originalZoomEventTypes];
-  const alreadyHandlesControlWheel = zoomEventTypes.some(
-    (binding) =>
-      binding?.eventType === Cesium.CameraEventType.WHEEL &&
-      binding?.modifier === Cesium.KeyboardEventModifier.CTRL,
-  );
-  const configuredZoomEventTypes = alreadyHandlesControlWheel
-    ? originalZoomEventTypes
-    : [
-        ...zoomEventTypes,
-        {
-          eventType: Cesium.CameraEventType.WHEEL,
-          modifier: Cesium.KeyboardEventModifier.CTRL,
-        },
-      ];
-  if (!alreadyHandlesControlWheel)
-    controller.zoomEventTypes = configuredZoomEventTypes;
-
-  const relayedEvents = new WeakSet();
-  const relayPinch = (event) => {
-    if (
-      !event.ctrlKey ||
-      relayedEvents.has(event) ||
-      event.deltaMode !== 0 ||
-      !Number.isFinite(event.deltaY) ||
-      event.deltaY === 0
-    )
-      return;
-    let relayed;
-    try {
-      relayed = createWheelEvent('wheel', {
-        deltaX: event.deltaX,
-        deltaY: boundedPinchDelta(event.deltaY),
-        deltaZ: event.deltaZ,
-        deltaMode: event.deltaMode,
-        screenX: event.screenX,
-        screenY: event.screenY,
-        clientX: event.clientX,
-        clientY: event.clientY,
-        ctrlKey: true,
-        bubbles: true,
-        cancelable: true,
-        view: globalThis.window,
-      });
-    } catch {
-      // The registered Ctrl+wheel binding can still consume the original.
-      return;
-    }
-    relayedEvents.add(relayed);
-    event.preventDefault();
-    event.stopPropagation();
-    canvas.dispatchEvent(relayed);
-  };
-  container.addEventListener('wheel', relayPinch, {
-    capture: true,
-    passive: false,
-  });
-
-  let active = true;
-  return () => {
-    if (!active) return;
-    active = false;
-    container.removeEventListener('wheel', relayPinch, true);
-    if (
-      !alreadyHandlesControlWheel &&
-      controller.zoomEventTypes === configuredZoomEventTypes
-    )
-      controller.zoomEventTypes = originalZoomEventTypes;
-  };
-}
-
 /** Cesium.Viewer options of the standard globe viewer. */
 export function applicationViewerOptions(creditContainer) {
   return {
diff --git a/src/render/cesium/ctrlWheelZoom.js b/src/render/cesium/ctrlWheelZoom.js
new file mode 100644
index 000000000..54ac3197a
--- /dev/null
+++ b/src/render/cesium/ctrlWheelZoom.js
@@ -0,0 +1,108 @@
+/**
+ * @module render/cesium/ctrlWheelZoom
+ * @description Ctrl+wheel zoom on a Cesium scene: a Ctrl+wheel binding among
+ * the camera controller's zoom inputs, and browser trackpad pinches (small
+ * pixel-mode Ctrl+wheel events) relayed at a usable speed.
+ */
+import * as Cesium from 'cesium';
+
+const PINCH_ZOOM_MULTIPLIER = 8;
+const MAX_PINCH_PIXEL_DELTA = 120;
+
+function boundedPinchDelta(delta) {
+  if (!Number.isFinite(delta) || delta === 0) return delta;
+  return (
+    Math.sign(delta) *
+    Math.min(Math.abs(delta) * PINCH_ZOOM_MULTIPLIER, MAX_PINCH_PIXEL_DELTA)
+  );
+}
+
+/**
+ * Add browser trackpad pinch to Cesium's zoom inputs and return its disposer.
+ * Browsers expose this gesture as a small pixel-mode Ctrl+wheel event.
+ */
+export function installTrackpadPinchZoom(
+  viewer,
+  { createWheelEvent = (type, init) => new WheelEvent(type, init) } = {},
+) {
+  const controller = viewer?.scene?.screenSpaceCameraController;
+  const container = viewer?.container;
+  const canvas = viewer?.canvas;
+  if (!controller || !container || !canvas)
+    throw new TypeError('A complete Cesium viewer is required');
+
+  const originalZoomEventTypes = controller.zoomEventTypes;
+  const zoomEventTypes = Array.isArray(originalZoomEventTypes)
+    ? originalZoomEventTypes
+    : originalZoomEventTypes === undefined
+      ? []
+      : [originalZoomEventTypes];
+  const alreadyHandlesControlWheel = zoomEventTypes.some(
+    (binding) =>
+      binding?.eventType === Cesium.CameraEventType.WHEEL &&
+      binding?.modifier === Cesium.KeyboardEventModifier.CTRL,
+  );
+  const configuredZoomEventTypes = alreadyHandlesControlWheel
+    ? originalZoomEventTypes
+    : [
+        ...zoomEventTypes,
+        {
+          eventType: Cesium.CameraEventType.WHEEL,
+          modifier: Cesium.KeyboardEventModifier.CTRL,
+        },
+      ];
+  if (!alreadyHandlesControlWheel)
+    controller.zoomEventTypes = configuredZoomEventTypes;
+
+  const relayedEvents = new WeakSet();
+  const relayPinch = (event) => {
+    if (
+      !event.ctrlKey ||
+      relayedEvents.has(event) ||
+      event.deltaMode !== 0 ||
+      !Number.isFinite(event.deltaY) ||
+      event.deltaY === 0
+    )
+      return;
+    let relayed;
+    try {
+      relayed = createWheelEvent('wheel', {
+        deltaX: event.deltaX,
+        deltaY: boundedPinchDelta(event.deltaY),
+        deltaZ: event.deltaZ,
+        deltaMode: event.deltaMode,
+        screenX: event.screenX,
+        screenY: event.screenY,
+        clientX: event.clientX,
+        clientY: event.clientY,
+        ctrlKey: true,
+        bubbles: true,
+        cancelable: true,
+        view: globalThis.window,
+      });
+    } catch {
+      // The registered Ctrl+wheel binding can still consume the original.
+      return;
+    }
+    relayedEvents.add(relayed);
+    event.preventDefault();
+    event.stopPropagation();
+    canvas.dispatchEvent(relayed);
+  };
+  container.addEventListener('wheel', relayPinch, {
+    capture: true,
+    passive: false,
+  });
+
+  let active = true;
+  return () => {
+    if (!active) return;
+    active = false;
+    container.removeEventListener('wheel', relayPinch, true);
+    if (
+      !alreadyHandlesControlWheel &&
+      controller.zoomEventTypes === configuredZoomEventTypes
+    )
+      controller.zoomEventTypes = originalZoomEventTypes;
+  };
+}