Honor user's reduced motion preference for backdrop animations

Disable scroll parallax and autozoom if the userhas enabled a setting
on their device to minimize the amount of non-essential motion.

entry_types/scrolled/package/spec/frontend/features/backdropAnimationEffects-spec.js

@@ -145,6 +145,33 @@ describe('backdrop animation effects', () => {
     expect(animateMock).not.toHaveBeenCalled();
   });
 
+  it('neither calls animate nor sets up view timeline when reduced motion is preferred', () => {
+    window.matchMedia.mockPrefersReducedMotion();
+
+    renderEntry({
+      seed: {
+        imageFiles: [{permaId: 100}],
+        sections: [
+          {
+            permaId: 1,
+            configuration: {
+              backdrop: {image: 100},
+              backdropEffects: [
+                {
+                  name: 'scrollParallax',
+                  value: 40
+                }
+              ]
+            }
+          }
+        ]
+      }
+    });
+
+    expect(viewTimelines.length).toEqual(0);
+    expect(animateMock).not.toHaveBeenCalled();
+  });
+
   it('supports auto zoom', () => {
     const {getSectionByPermaId} = renderEntry({
       seed: {
@@ -245,4 +272,32 @@ describe('backdrop animation effects', () => {
 
     expect(animateMock).not.toHaveBeenCalled();
   });
+
+  it('does not autozoom if reduced motion is preferred', () => {
+    window.matchMedia.mockPrefersReducedMotion();
+
+    const {getSectionByPermaId} = renderEntry({
+      seed: {
+        imageFiles: [{permaId: 100}],
+        sections: [
+          {
+            permaId: 1,
+            configuration: {
+              backdrop: {image: 100},
+              backdropEffects: [
+                {
+                  name: 'autoZoom',
+                  value: 50
+                }
+              ]
+            }
+          }
+        ]
+      }
+    });
+
+    getSectionByPermaId(1).simulateScrollingIntoView();
+
+    expect(animateMock).not.toHaveBeenCalled();
+  });
 });

entry_types/scrolled/package/spec/frontend/v1/Backdrop/Effects-spec.js

@@ -36,8 +36,4 @@ describe('Backdrop Effects getFilter', () => {
 
     expect(result).toEqual('grayscale(20%)');
   });
-
-  it('sets up scroll animation  ', () => {
-
-  });
 });

entry_types/scrolled/package/spec/support/matchMediaStub.js

@@ -1,4 +1,5 @@
 let mockOrientation;
+let mockPrefersReducedMotion;
 
 Object.defineProperty(window, 'matchMedia', {
   writable: true,
@@ -17,6 +18,13 @@ Object.defineProperty(window, 'matchMedia', {
         matches: mockOrientation !== 'portrait'
       };
     }
+    else if (query === '(prefers-reduced-motion)') {
+      return {
+        addEventListener: jest.fn(),
+        removeEventListener: jest.fn(),
+        matches: mockPrefersReducedMotion
+      };
+    }
     else {
       return {
         addEventListener: jest.fn(),
@@ -27,6 +35,10 @@ Object.defineProperty(window, 'matchMedia', {
   })
 });
 
-beforeEach(() => mockOrientation = 'landscape');
+beforeEach(() => {
+  mockOrientation = 'landscape';
+  mockPrefersReducedMotion = false;
+});
 
 window.matchMedia.mockPortrait = () => mockOrientation = 'portrait';
+window.matchMedia.mockPrefersReducedMotion = () => mockPrefersReducedMotion = true;

entry_types/scrolled/package/src/frontend/SectionViewTimelineProvider.js

@@ -1,5 +1,7 @@
 import React, {useContext, useEffect, useState, useRef} from 'react';
 
+import {prefersReducedMotion} from './prefersReducedMotion';
+
 const SectionViewTimelineContext = React.createContext();
 
 export function SectionViewTimelineProvider({backdrop, children}) {
@@ -9,7 +11,7 @@ export function SectionViewTimelineProvider({backdrop, children}) {
   const isNeeded = backdrop?.effects?.some(effect => effect.name === 'scrollParallax');
 
   useEffect(() => {
-    if (!isNeeded || !window.ViewTimeline) {
+    if (!isNeeded || !window.ViewTimeline || prefersReducedMotion()) {
       return;
     }
 

entry_types/scrolled/package/src/frontend/prefersReducedMotion.js

@@ -0,0 +1,3 @@
+export function prefersReducedMotion() {
+  return window.matchMedia('(prefers-reduced-motion)').matches;
+}

entry_types/scrolled/package/src/frontend/v1/Backdrop/Effects.js

@@ -6,6 +6,7 @@ import styles from '../../Backdrop.module.css';
 import {useSectionViewTimeline} from '../../SectionViewTimelineProvider';
 import {useSectionLifecycle} from '../../useSectionLifecycle';
 import {useIsStaticPreview} from '../../useScrollPositionLifecycle';
+import {prefersReducedMotion} from '../../prefersReducedMotion';
 
 export function Effects({file, children}) {
   const ref = useRef();
@@ -45,7 +46,7 @@ export function Effects({file, children}) {
   const y = file?.motifArea ? 50 - (file.motifArea.top + file.motifArea.height / 2) : 0;
 
   useIsomorphicLayoutEffect(() => {
-    if (autoZoomValue && isVisible) {
+    if (autoZoomValue && isVisible && !prefersReducedMotion()) {
       const animation = ref.current.animate(
         {
           transform: [