mediawiki-skins-Vector/resources/skins.vector.js/setupIntersectionObservers.js

// Enable Vector features limited to ES6 browse
const
	stickyHeader = require( './stickyHeader.js' ),
	scrollObserver = require( './scrollObserver.js' ),
	initExperiment = require( './AB.js' ),
	initSectionObserver = require( './sectionObserver.js' ),
	initTableOfContents = require( './tableOfContents.js' ),
	pinnableElement = require( './pinnableElement.js' ),
	popupNotification = require( './popupNotification.js' ),
	features = require( './features.js' ),
	deferUntilFrame = require( './deferUntilFrame.js' ),
	ABTestConfig = require( /** @type {string} */ ( './activeABTest.json' ) ),
	STICKY_HEADER_VISIBLE_CLASS = 'vector-sticky-header-visible',
	TOC_ID = 'vector-toc',
	BODY_CONTENT_ID = 'bodyContent',
	HEADLINE_SELECTOR = '.mw-headline',
	TOC_SECTION_ID_PREFIX = 'toc-',
	PAGE_TITLE_INTERSECTION_CLASS = 'vector-below-page-title';

const belowDesktopMedia = window.matchMedia( '(max-width: 999px)' );

/**
 * @callback OnIntersection
 * @param {HTMLElement} element The section that triggered the new intersection change.
 */

/**
 * @ignore
 * @param {Function} changeActiveSection
 * @return {OnIntersection}
 */
const getHeadingIntersectionHandler = ( changeActiveSection ) =>
	/**
	 * @param {HTMLElement} section
	 */
	// eslint-disable-next-line implicit-arrow-linebreak
	( section ) => {
		const headline = section.classList.contains( 'mw-body-content' ) ?
			section :
			section.querySelector( HEADLINE_SELECTOR );
		if ( headline ) {
			changeActiveSection( `${ TOC_SECTION_ID_PREFIX }${ headline.id }` );
		}
	};

/**
 * Initialize sticky header AB tests and determine whether to show the sticky header
 * based on which buckets the user is in.
 *
 * @typedef {Object} InitStickyHeaderABTests
 * @property {boolean} showStickyHeader - Should the sticky header be shown
 * @param {ABTestConfig} abConfig
 * @param {boolean} isStickyHeaderFeatureAllowed and the user is logged in
 * @param {function(ABTestConfig): initExperiment.WebABTest} getEnabledExperiment
 * @return {InitStickyHeaderABTests}
 */
function initStickyHeaderABTests( abConfig, isStickyHeaderFeatureAllowed, getEnabledExperiment ) {
	let showStickyHeader = isStickyHeaderFeatureAllowed,
		stickyHeaderExperiment;

	// One of the sticky header AB tests is specified in the config
	const abTestName = abConfig.name,
		isStickyHeaderExperiment = abTestName === stickyHeader.STICKY_HEADER_EXPERIMENT_NAME;

	// Determine if user is eligible for sticky header AB test
	if (
		isStickyHeaderFeatureAllowed && // The sticky header can be shown on the page
		abConfig.enabled && // An AB test config is enabled
		isStickyHeaderExperiment // The AB test is one of the sticky header experiments
	) {
		// If eligible, initialize the AB test
		stickyHeaderExperiment = getEnabledExperiment( abConfig );

		// If running initial or edit AB test, show sticky header to treatment groups
		// only. Unsampled and control buckets do not see sticky header.
		if ( abTestName === stickyHeader.STICKY_HEADER_EXPERIMENT_NAME ) {
			showStickyHeader = stickyHeaderExperiment.isInTreatmentBucket();
		}
	}

	return {
		showStickyHeader
	};
}

/*
 * Updates TOC's location in the DOM (in sidebar or sticky header)
 * depending on if the TOC is collapsed and if the sticky header is visible
 *
 * @return {void}
 */
const updateTocLocation = () => {
	const isPinned = features.isEnabled( 'toc-pinned' );
	const isStickyHeaderVisible = document.body.classList.contains( STICKY_HEADER_VISIBLE_CLASS );
	const isBelowDesktop = belowDesktopMedia.matches;

	const pinnedContainerId = 'vector-toc-pinned-container';
	const stickyHeaderUnpinnedContainerId = 'vector-sticky-header-toc-unpinned-container';
	const pageTitlebarUnpinnedContainerId = 'vector-page-titlebar-toc-unpinned-container';

	let newContainerId = '';
	if ( isPinned ) {
		if ( isBelowDesktop ) {
			// Automatically move the ToC into the page titlebar when pinned on smaller resolutions
			newContainerId = pageTitlebarUnpinnedContainerId;
		} else {
			newContainerId = pinnedContainerId;
		}
	} else {
		if ( isStickyHeaderVisible && !isBelowDesktop ) {
			newContainerId = stickyHeaderUnpinnedContainerId;
		} else {
			newContainerId = pageTitlebarUnpinnedContainerId;
		}
	}

	pinnableElement.movePinnableElement( TOC_ID, newContainerId );
};

/**
 * Return the computed value of the `scroll-margin-top` CSS property of the document element
 * which is also used for the scroll intersection threshold (T317661).
 *
 * @return {number} Value of scroll-margin-top OR 75 if falsy.
 * 75 derived from @scroll-padding-top LESS variable
 * https://gerrit.wikimedia.org/r/c/mediawiki/skins/Vector/+/894696/3/resources/common/variables.less ?
 */
function getDocumentScrollPaddingTop() {
	const defaultScrollPaddingTop = 75;
	const documentStyles = getComputedStyle( document.documentElement );
	const scrollPaddingTopString = documentStyles.getPropertyValue( 'scroll-padding-top' );
	return ( parseInt( scrollPaddingTopString, 10 ) || defaultScrollPaddingTop );
}

/**
 * @param {HTMLElement|null} tocElement
 * @param {HTMLElement|null} bodyContent
 * @param {initSectionObserver} initSectionObserverFn
 * @return {tableOfContents|null}
 */
const setupTableOfContents = ( tocElement, bodyContent, initSectionObserverFn ) => {
	if ( !(
		tocElement &&
		bodyContent
	) ) {
		return null;
	}

	const handleTocSectionChange = () => {
		// eslint-disable-next-line no-use-before-define
		sectionObserver.pause();

		// T297614: We want the link that the user has clicked inside the TOC or the
		// section that corresponds to the hashchange event to be "active" (e.g.
		// bolded) regardless of whether the browser's scroll position corresponds
		// to that section. Therefore, we need to temporarily ignore section
		// observer until the browser has finished scrolling to the section (if
		// needed).
		//
		// However, because the scroll event happens asynchronously after the user
		// clicks on a link and may not even happen at all (e.g. the user has
		// scrolled all the way to the bottom and clicks a section that is already
		// in the viewport), determining when we should resume section observer is a
		// bit tricky.
		//
		// Because a scroll event may not even be triggered after clicking the link,
		// we instead allow the browser to perform a maximum number of repaints
		// before resuming sectionObserver. Per T297614#7687656, Firefox 97.0 wasn't
		// consistently activating the table of contents section that the user
		// clicked even after waiting 2 frames. After further investigation, it
		// sometimes waits up to 3 frames before painting the new scroll position so
		// we have that as the limit.
		deferUntilFrame( () => {
			// eslint-disable-next-line no-use-before-define
			sectionObserver.resume();
		}, 3 );
	};

	const tableOfContents = initTableOfContents( {
		container: tocElement,
		onHeadingClick: handleTocSectionChange,
		onHashChange: handleTocSectionChange,
		onTogglePinned: () => {
			updateTocLocation();
			pinnableElement.setFocusAfterToggle( TOC_ID );
			if ( !features.isEnabled( 'toc-pinned' ) ) {
				const isStickyHeaderVisible = document.body.classList
					.contains( STICKY_HEADER_VISIBLE_CLASS );
				const containerSelector = !isStickyHeaderVisible ?
					'.vector-page-titlebar .vector-toc-landmark' : '#vector-sticky-header .vector-toc-landmark';
				const container = /** @type {HTMLElement} */(
					document.querySelector( containerSelector )
				);
				if ( container ) {
					const containerId = !isStickyHeaderVisible ? 'toc-page-titlebar' : 'toc-sticky-header';
					popupNotification.add( container, mw.message( 'vector-toc-unpinned-popup' ).text(), containerId )
						.then( ( popupWidget ) => {
							if ( popupWidget ) {
								popupNotification.show( popupWidget );
							}
						} );
				}
			}

		}
	} );
	const headingSelector = [
		'h1', 'h2', 'h3', 'h4', 'h5', 'h6'
	].map( ( tag ) => `.mw-parser-output ${ tag }` ).join( ',' );
	const elements = () => bodyContent.querySelectorAll( `${ headingSelector }, .mw-body-content` );

	const sectionObserver = initSectionObserverFn( {
		elements: elements(),
		topMargin: getDocumentScrollPaddingTop(),
		onIntersection: getHeadingIntersectionHandler( tableOfContents.changeActiveSection )
	} );
	const updateElements = () => {
		sectionObserver.resume();
		sectionObserver.setElements( elements() );
	};
	mw.hook( 've.activationStart' ).add( () => {
		sectionObserver.pause();
	} );
	mw.hook( 'wikipage.tableOfContents' ).add( ( sections ) => {
		tableOfContents.reloadTableOfContents( sections ).then( () => {
			/**
			 * @stable for use in gadgets and extensions
			 * @since 1.40
			 */
			mw.hook( 'wikipage.tableOfContents.vector' ).fire( sections );
			updateElements();
		} );
	} );
	mw.hook( 've.deactivationComplete' ).add( updateElements );

	const setInitialActiveSection = () => {
		const hash = location.hash.slice( 1 );
		// If hash fragment is blank, determine the active section with section
		// observer.
		if ( hash === '' ) {
			sectionObserver.calcIntersection();
			return;
		}

		// T325086: If hash fragment is present and corresponds to a toc section,
		// expand the section.
		const hashSection = /** @type {HTMLElement|null} */ ( mw.util.getTargetFromFragment( `${ TOC_SECTION_ID_PREFIX }${ hash }` ) );
		if ( hashSection ) {
			tableOfContents.expandSection( hashSection.id );
		}

		// T325086: If hash fragment corresponds to a section AND the user is at
		// bottom of page, activate the section. Otherwise, use section observer to
		// calculate the active section.
		//
		// Note that even if a hash fragment is present, it's possible for the
		// browser to scroll to a position that is different from the position of
		// the section that corresponds to the hash fragment. This can happen when
		// the browser remembers a prior scroll position after refreshing the page,
		// for example.
		if (
			hashSection &&
			Math.round( window.innerHeight + window.scrollY ) >= document.body.scrollHeight
		) {
			tableOfContents.changeActiveSection( hashSection.id );
		} else {
			// Fallback to section observer's calculation for the active section.
			sectionObserver.calcIntersection();
		}
	};

	setInitialActiveSection();

	return tableOfContents;
};

/**
 * @return {void}
 */
const main = () => {
	const isIntersectionObserverSupported = 'IntersectionObserver' in window;

	//
	//  Table of contents
	//
	const tocElement = document.getElementById( TOC_ID );
	const bodyContent = document.getElementById( BODY_CONTENT_ID );

	const isToCUpdatingAllowed = isIntersectionObserverSupported &&
		window.requestAnimationFrame;
	const tableOfContents = isToCUpdatingAllowed ?
		setupTableOfContents( tocElement, bodyContent, initSectionObserver ) : null;

	//
	// Sticky header
	//
	const
		stickyHeaderElement = document.getElementById( stickyHeader.STICKY_HEADER_ID ),
		stickyIntersection = document.getElementById( stickyHeader.FIRST_HEADING_ID ),
		userLinksDropdown = document.getElementById( stickyHeader.USER_LINKS_DROPDOWN_ID ),
		allowedNamespace = stickyHeader.isAllowedNamespace( mw.config.get( 'wgNamespaceNumber' ) ),
		allowedAction = stickyHeader.isAllowedAction( mw.config.get( 'wgAction' ) );

	const isStickyHeaderAllowed =
		!!stickyHeaderElement &&
		!!stickyIntersection &&
		!!userLinksDropdown &&
		allowedNamespace &&
		allowedAction &&
		isIntersectionObserverSupported;

	const { showStickyHeader } = initStickyHeaderABTests(
		ABTestConfig,
		isStickyHeaderAllowed && !mw.user.isAnon(),
		( config ) => initExperiment(
			config,
			String( mw.user.getId() )
		)
	);

	// Set up intersection observer for page title
	// Used to show/hide sticky header and add class used by collapsible TOC (T307900)
	const observer = scrollObserver.initScrollObserver(
		() => {
			if ( isStickyHeaderAllowed && showStickyHeader ) {
				stickyHeader.show();
				updateTocLocation();
			}
			document.body.classList.add( PAGE_TITLE_INTERSECTION_CLASS );
			if ( tableOfContents ) {
				tableOfContents.updateTocToggleStyles( true );
			}
			scrollObserver.firePageTitleScrollHook( 'down' );
		},
		() => {
			if ( isStickyHeaderAllowed && showStickyHeader ) {
				stickyHeader.hide();
				updateTocLocation();
			}
			document.body.classList.remove( PAGE_TITLE_INTERSECTION_CLASS );
			if ( tableOfContents ) {
				tableOfContents.updateTocToggleStyles( false );
			}
			scrollObserver.firePageTitleScrollHook( 'up' );
		}
	);

	// Handle toc location when sticky header is hidden on lower viewports
	belowDesktopMedia.onchange = () => {
		updateTocLocation();
	};

	updateTocLocation();

	if ( !showStickyHeader ) {
		stickyHeader.hide();
	}

	if ( isStickyHeaderAllowed && showStickyHeader ) {
		stickyHeader.initStickyHeader( {
			header: stickyHeaderElement,
			userLinksDropdown,
			observer,
			stickyIntersection
		} );
	} else if ( stickyIntersection ) {
		observer.observe( stickyIntersection );
	}
};

module.exports = {
	main,
	test: {
		setupTableOfContents,
		initStickyHeaderABTests,
		getHeadingIntersectionHandler
	}
};