Disable animations when user prefers reduced motion

When browser preference for reduced motion is enabled:
* Disables bolding of table of contents
* Disables sticky header transition

Bug: T254399
Change-Id: I8ef9e59b258fed977ce370da352b1924832d842b

bundlesize.config.json

@@ -9,7 +9,7 @@
 	},
 	{
 		"resourceModule": "skins.vector.legacy.js",
-		"maxSize": "2 kB"
+		"maxSize": "2.2 kB"
 	},
 	{
 		"resourceModule": "skins.vector.search",

resources/skins.vector.es6/tableOfContents.js

@@ -53,6 +53,15 @@ module.exports = function tableOfContents( props ) {
 		};
 	}
 
+	/**
+	 * Does the user prefer reduced motion?
+	 *
+	 * @return {boolean}
+	 */
+	const prefersReducedMotion = () => {
+		return window.matchMedia( '(prefers-reduced-motion: reduce)' ).matches;
+	};
+
 	/**
 	 * Sets an `ACTIVE_SECTION_CLASS` on the element with an id that matches `id`.
 	 * If the element is not a top level heading (e.g. element with the
@@ -78,7 +87,12 @@ module.exports = function tableOfContents( props ) {
 
 		const topSection = /** @type {HTMLElement} */ ( selectedTocSection.closest( `.${PARENT_SECTION_CLASS}` ) );
 
-		if ( selectedTocSection === topSection ) {
+		// The bolding of sections is arguably not "motion", however does provide a distraction to readers
+		// who are scrolling by visibly changing the table of contents. This can be removed if someone has
+		// a strong argument for why this should not be considered motion.
+		if ( prefersReducedMotion() ) {
+			return;
+		} else if ( selectedTocSection === topSection ) {
 			activeTopSection = topSection;
 			activeTopSection.classList.add( ACTIVE_SECTION_CLASS );
 		} else {
@@ -144,8 +158,7 @@ module.exports = function tableOfContents( props ) {
 				Math.min( containerRect.bottom, window.innerHeight );
 
 			// Respect 'prefers-reduced-motion' user preference
-			const mediaQuery = window.matchMedia( '(prefers-reduced-motion: reduce)' );
-			const scrollBehavior = ( !mediaQuery || !mediaQuery.matches ) ? 'smooth' : undefined;
+			const scrollBehavior = prefersReducedMotion() ? 'smooth' : undefined;
 
 			// Manually increment and decrement TOC scroll rather than using scrollToView
 			// in order to account for threshold

resources/skins.vector.styles/components/StickyHeader.less

@@ -23,11 +23,6 @@
 	justify-content: space-between;
 	box-sizing: border-box;
 
-	// If the user has expressed their preference for reduced motion, then disable animation for the sticky header.
-	@media ( prefers-reduced-motion: reduce ) {
-		transition: none;
-	}
-
 	@media ( min-width: @width-breakpoint-desktop ) {
 		padding: 6px 25px;
 	}

resources/skins.vector.styles/skin.less

@@ -29,3 +29,23 @@
 @media print {
 	@import './layouts/print.less';
 }
+
+/**
+ * Respect users who prefer reduced motion.
+ * This code can be removed if and when it is upstreamed to ResourceLoaderSkinModule
+ * (see T254399).
+ */
+/* stylelint-disable declaration-no-important, time-min-milliseconds */
+@media ( prefers-reduced-motion: reduce ) {
+	*,
+	:before,
+	:after {
+		animation-delay: -1ms !important; /* 1 */
+		animation-duration: 1ms !important; /* 1 */
+		animation-iteration-count: 1 !important; /* 1 */
+		background-attachment: initial !important; /* 2 */
+		scroll-behavior: auto !important; /* 3 */
+		transition-delay: 0s !important; /* 4 */
+		transition-duration: 0s !important; /* 4 */
+	}
+}

tests/jest/tableOfContents.test.js

@@ -92,6 +92,11 @@ function mount( templateProps = {} ) {
 }
 
 describe( 'Table of contents', () => {
+	beforeEach( () => {
+		// @ts-ignore
+		global.window.matchMedia = jest.fn( () => ( {} ) );
+	} );
+
 	describe( 'renders', () => {
 		test( 'when `vector-is-collapse-sections-enabled` is false', () => {
 			const toc = mount();