polygerrit-ui/app/elements/shared/gr-cursor-manager/gr-cursor-manager.ts - gerrit - Git at Google

 /**
  * @license
  * Copyright (C) 2016 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  * http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
 import {BehaviorSubject} from 'rxjs';
 import {AbortStop, CursorMoveResult, Stop} from '../../../api/core';
 import {ScrollMode} from '../../../constants/constants';

 /**
  * Type guard and checker to check if a stop can be targeted.
  * Abort stops cannot be targeted.
  */
 export function isTargetable(stop: Stop): stop is HTMLElement {
   return !(stop instanceof AbortStop);
 }

 export class GrCursorManager {
   get target(): HTMLElement | null {
     return this.targetSubject.getValue();
   }

   set target(target: HTMLElement | null) {
     this.targetSubject.next(target);
     this._scrollToTarget();
   }

   private targetSubject = new BehaviorSubject<HTMLElement | null>(null);

   target$ = this.targetSubject.asObservable();

   /**
    * The height of content intended to be included with the target.
    */
   _targetHeight: number | null = null;

   /**
    * The index of the current target (if any). -1 otherwise.
    */
   index = -1;

   /**
    * The class to apply to the current target. Use null for no class.
    */
   cursorTargetClass: string | null = null;

   /**
    * The scroll behavior for the cursor. Values are 'never' and
    * 'keep-visible'. 'keep-visible' will only scroll if the cursor is beyond
    * the viewport.
    * TODO (beckysiegel) figure out why it can be undefined
    *
    * @type {string|undefined}
    */
   scrollMode: string = ScrollMode.NEVER;

   /**
    * When true, will call element.focus() during scrolling.
    */
   focusOnMove = false;

   set stops(stops: Stop[]) {
     this.stopsInternal = stops;
     this._updateIndex();
   }

   get stops(): Stop[] {
     return this.stopsInternal;
   }

   private stopsInternal: Stop[] = [];

   /** Only non-AbortStop stops. */
   get targetableStops(): HTMLElement[] {
     return this.stops.filter(isTargetable);
   }

   /**
    * Move the cursor forward. Clipped to the ends of the stop list.
    *
    * @param options.filter Skips any stops for which filter returns false.
    * @param options.getTargetHeight Optional function to calculate the
    *    height of the target's 'section'. The height of the target itself is
    *    sometimes different, used by the diff cursor.
    * @param options.clipToTop When none of the next indices match, move
    *     back to first instead of to last.
    * @return If a move was performed or why not.
    * @private
    */
   next(
     options: {
       filter?: (stop: HTMLElement) => boolean;
       getTargetHeight?: (target: HTMLElement) => number;
       clipToTop?: boolean;
     } = {}
   ): CursorMoveResult {
     return this._moveCursor(1, options);
   }

   previous(
     options: {
       filter?: (stop: HTMLElement) => boolean;
     } = {}
   ): CursorMoveResult {
     return this._moveCursor(-1, options);
   }

   /**
    * Move the cursor to the row which is the closest to the viewport center
    * in vertical direction.
    * The method uses IntersectionObservers API. If browser
    * doesn't support this API the method does nothing
    *
    * @param filter Skips any stops for which filter returns false.
    */
   async moveToVisibleArea(filter?: (el: Element) => boolean) {
     const centerMostStop = await this.getCenterMostStop(filter);
     // In most cases the target is visible, so scroll is not
     // needed. But in rare cases the target can become invisible
     // at this point (due to some scrolling in window).
     // To avoid jumps set noScroll options.
     if (centerMostStop) {
       this.setCursor(centerMostStop, true);
     }
   }

   private async getCenterMostStop(
     filter?: (el: Element) => boolean
   ): Promise<HTMLElement | undefined> {
     const visibleEntries = await this.getVisibleEntries(filter);
     const windowCenter = Math.round(window.innerHeight / 2);

     let centerMostStop: HTMLElement | undefined = undefined;
     let minDistanceToCenter = Number.MAX_VALUE;

     for (const entry of visibleEntries) {
       // We are just using the entries here, because entry.boundingClientRect
       // is already computed, but entry.target.getBoundingClientRect() should
       // actually yield the same result.
       const center =
         entry.boundingClientRect.top +
         Math.round(entry.boundingClientRect.height / 2);
       const distanceToWindowCenter = Math.abs(center - windowCenter);
       if (distanceToWindowCenter < minDistanceToCenter) {
         // entry.target comes from the filteredStops array,
         // hence it is an HTMLElement
         centerMostStop = entry.target as HTMLElement;
         minDistanceToCenter = distanceToWindowCenter;
       }
     }
     return centerMostStop;
   }

   private async getVisibleEntries(
     filter?: (el: Element) => boolean
   ): Promise<IntersectionObserverEntry[]> {
     if (!this._isIntersectionObserverSupported()) {
       throw new Error('Intersection observing not supported');
     }
     if (!this.stops) {
       return [];
     }
     const filteredStops = filter
       ? this.targetableStops.filter(filter)
       : this.targetableStops;
     return new Promise(resolve => {
       let unobservedCount = filteredStops.length;
       const visibleEntries: IntersectionObserverEntry[] = [];
       const observer = new IntersectionObserver(entries => {
         visibleEntries.push(
           ...entries
             // In Edge it is recommended to use intersectionRatio instead of
             // isIntersecting.
             .filter(
               entry => entry.isIntersecting || entry.intersectionRatio > 0
             )
         );

         // This callback is called for the first time immediately.
         // Typically it gets all observed stops at once, but
         // sometimes can get them in several chunks.
         for (const entry of entries) {
           observer.unobserve(entry.target);
         }
         unobservedCount -= entries.length;
         if (unobservedCount === 0) {
           resolve(visibleEntries);
         }
       });
       for (const stop of filteredStops) {
         observer.observe(stop);
       }
     });
   }

   _isIntersectionObserverSupported() {
     // The copy of this method exists in gr-app-element.js under the
     // name _isCursorManagerSupportMoveToVisibleLine
     // If you update this method, you must update gr-app-element.js
     // as well.
     return 'IntersectionObserver' in window;
   }

   /**
    * Set the cursor to an arbitrary stop - if the given element is not one of
    * the stops, unset the cursor.
    *
    * @param noScroll prevent any potential scrolling in response
    * setting the cursor.
    */
   setCursor(element: HTMLElement, noScroll?: boolean) {
     if (!this.targetableStops.includes(element)) {
       this.unsetCursor();
       return;
     }
     let behavior;
     if (noScroll) {
       behavior = this.scrollMode;
       this.scrollMode = ScrollMode.NEVER;
     }

     this.unsetCursor();
     this.target = element;
     this._updateIndex();
     this._decorateTarget();

     if (noScroll && behavior) {
       this.scrollMode = behavior;
     }
   }

   unsetCursor() {
     this._unDecorateTarget();
     this.index = -1;
     this.target = null;
     this._targetHeight = null;
   }

   /** Returns true if there are no stops, or we are on the first stop. */
   isAtStart(): boolean {
     return this.stops.length === 0 || this.index === 0;
   }

   /** Returns true if there are no stops, or we are on the last stop. */
   isAtEnd(): boolean {
     return this.stops.length === 0 || this.index === this.stops.length - 1;
   }

   moveToStart() {
     if (this.stops.length) {
       this.setCursorAtIndex(0);
     }
   }

   moveToEnd() {
     if (this.stops.length) {
       this.setCursorAtIndex(this.stops.length - 1);
     }
   }

   setCursorAtIndex(index: number, noScroll?: boolean) {
     const stop = this.stops[index];
     if (isTargetable(stop)) {
       this.setCursor(stop, noScroll);
     }
   }

   /**
    * Move the cursor forward or backward by delta. Clipped to the beginning or
    * end of stop list.
    *
    * @param delta either -1 or 1.
    * @param options.abort Will abort moving the cursor when encountering a
    *    stop for which this condition is met. Will abort even if the stop
    *    would have been filtered
    * @param options.filter Will keep going and skip any stops for which this
    *    condition is not met.
    * @param options.getTargetHeight Optional function to calculate the
    * height of the target's 'section'. The height of the target itself is
    * sometimes different, used by the diff cursor.
    * @param options.clipToTop When none of the next indices match, move
    * back to first instead of to last.
    * @return  If a move was performed or why not.
    * @private
    */
   _moveCursor(
     delta: number,
     {
       filter,
       getTargetHeight,
       clipToTop,
     }: {
       filter?: (stop: HTMLElement) => boolean;
       getTargetHeight?: (target: HTMLElement) => number;
       clipToTop?: boolean;
     } = {}
   ): CursorMoveResult {
     if (!this.stops.length) {
       this.unsetCursor();
       return CursorMoveResult.NO_STOPS;
     }

     let newIndex = this.index;
     // If the cursor is not yet set and we are going backwards, start at the
     // back.
     if (this.index === -1 && delta < 0) {
       newIndex = this.stops.length;
     }

     let clipped = false;
     let newStop: Stop;
     do {
       newIndex += delta;
       if (
         (delta > 0 && newIndex >= this.stops.length) ||
         (delta < 0 && newIndex < 0)
       ) {
         newIndex = delta < 0 || clipToTop ? 0 : this.stops.length - 1;
         newStop = this.stops[newIndex];
         clipped = true;
         break;
       }
       // Sadly needed so that type narrowing understands that this.stops[newIndex] is
       // targetable after I have checked that.
       newStop = this.stops[newIndex];
     } while (isTargetable(newStop) && filter && !filter(newStop));

     if (!isTargetable(newStop)) {
       return CursorMoveResult.ABORTED;
     }

     this._unDecorateTarget();

     this.index = newIndex;
     this.target = newStop;

     if (getTargetHeight) {
       this._targetHeight = getTargetHeight(this.target);
     } else {
       this._targetHeight = this.target.scrollHeight;
     }

     if (this.focusOnMove) {
       this.target.focus();
     }

     this._decorateTarget();

     return clipped ? CursorMoveResult.CLIPPED : CursorMoveResult.MOVED;
   }

   _decorateTarget() {
     if (this.target && this.cursorTargetClass) {
       this.target.classList.add(this.cursorTargetClass);
     }
   }

   _unDecorateTarget() {
     if (this.target && this.cursorTargetClass) {
       this.target.classList.remove(this.cursorTargetClass);
     }
   }

   _updateIndex() {
     if (!this.target) {
       this.index = -1;
       return;
     }

     const newIndex = this.stops.indexOf(this.target);
     if (newIndex === -1) {
       this.unsetCursor();
     } else {
       this.index = newIndex;
     }
   }

   /**
    * Calculate where the element is relative to the window.
    *
    * @param target Target to scroll to.
    * @return Distance to top of the target.
    */
   _getTop(target: HTMLElement) {
     let top: number = target.offsetTop;
     for (
       let offsetParent = target.offsetParent;
       offsetParent;
       offsetParent = (offsetParent as HTMLElement).offsetParent
     ) {
       top += (offsetParent as HTMLElement).offsetTop;
     }
     return top;
   }

   _targetIsVisible(top: number) {
     return (
       this.scrollMode === ScrollMode.KEEP_VISIBLE &&
       top > window.pageYOffset &&
       top < window.pageYOffset + window.innerHeight
     );
   }

   _calculateScrollToValue(top: number, target: HTMLElement) {
     return top + -window.innerHeight / 3 + target.offsetHeight / 2;
   }

   _scrollToTarget() {
     if (!this.target || this.scrollMode === ScrollMode.NEVER) {
       return;
     }

     const top = this._getTop(this.target);
     const bottomIsVisible = this._targetHeight
       ? this._targetIsVisible(top + this._targetHeight)
       : true;
     const scrollToValue = this._calculateScrollToValue(top, this.target);

     if (this._targetIsVisible(top)) {
       // Don't scroll if either the bottom is visible or if the position that
       // would get scrolled to is higher up than the current position. This
       // would cause less of the target content to be displayed than is
       // already.
       if (bottomIsVisible || scrollToValue < window.scrollY) {
         return;
       }
     }

     // Scroll the element to the middle of the window. Dividing by a third
     // instead of half the inner height feels a bit better otherwise the
     // element appears to be below the center of the window even when it
     // isn't.
     window.scrollTo(window.scrollX, scrollToValue);
   }
 }
	/**
	* @license
	* Copyright (C) 2016 The Android Open Source Project
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/
	import {BehaviorSubject} from 'rxjs';
	import {AbortStop, CursorMoveResult, Stop} from '../../../api/core';
	import {ScrollMode} from '../../../constants/constants';

	/**
	* Type guard and checker to check if a stop can be targeted.
	* Abort stops cannot be targeted.
	*/
	export function isTargetable(stop: Stop): stop is HTMLElement {
	return !(stop instanceof AbortStop);
	}

	export class GrCursorManager {
	get target(): HTMLElement \| null {
	return this.targetSubject.getValue();
	}

	set target(target: HTMLElement \| null) {
	this.targetSubject.next(target);
	this._scrollToTarget();
	}

	private targetSubject = new BehaviorSubject<HTMLElement \| null>(null);

	target$ = this.targetSubject.asObservable();

	/**
	* The height of content intended to be included with the target.
	*/
	_targetHeight: number \| null = null;

	/**
	* The index of the current target (if any). -1 otherwise.
	*/
	index = -1;

	/**
	* The class to apply to the current target. Use null for no class.
	*/
	cursorTargetClass: string \| null = null;

	/**
	* The scroll behavior for the cursor. Values are 'never' and
	* 'keep-visible'. 'keep-visible' will only scroll if the cursor is beyond
	* the viewport.
	* TODO (beckysiegel) figure out why it can be undefined
	*
	* @type {string\|undefined}
	*/
	scrollMode: string = ScrollMode.NEVER;

	/**
	* When true, will call element.focus() during scrolling.
	*/
	focusOnMove = false;

	set stops(stops: Stop[]) {
	this.stopsInternal = stops;
	this._updateIndex();
	}

	get stops(): Stop[] {
	return this.stopsInternal;
	}

	private stopsInternal: Stop[] = [];

	/** Only non-AbortStop stops. */
	get targetableStops(): HTMLElement[] {
	return this.stops.filter(isTargetable);
	}

	/**
	* Move the cursor forward. Clipped to the ends of the stop list.
	*
	* @param options.filter Skips any stops for which filter returns false.
	* @param options.getTargetHeight Optional function to calculate the
	* height of the target's 'section'. The height of the target itself is
	* sometimes different, used by the diff cursor.
	* @param options.clipToTop When none of the next indices match, move
	* back to first instead of to last.
	* @return If a move was performed or why not.
	* @private
	*/
	next(
	options: {
	filter?: (stop: HTMLElement) => boolean;
	getTargetHeight?: (target: HTMLElement) => number;
	clipToTop?: boolean;
	} = {}
	): CursorMoveResult {
	return this._moveCursor(1, options);
	}

	previous(
	options: {
	filter?: (stop: HTMLElement) => boolean;
	} = {}
	): CursorMoveResult {
	return this._moveCursor(-1, options);
	}

	/**
	* Move the cursor to the row which is the closest to the viewport center
	* in vertical direction.
	* The method uses IntersectionObservers API. If browser
	* doesn't support this API the method does nothing
	*
	* @param filter Skips any stops for which filter returns false.
	*/
	async moveToVisibleArea(filter?: (el: Element) => boolean) {
	const centerMostStop = await this.getCenterMostStop(filter);
	// In most cases the target is visible, so scroll is not
	// needed. But in rare cases the target can become invisible
	// at this point (due to some scrolling in window).
	// To avoid jumps set noScroll options.
	if (centerMostStop) {
	this.setCursor(centerMostStop, true);
	}
	}

	private async getCenterMostStop(
	filter?: (el: Element) => boolean
	): Promise<HTMLElement \| undefined> {
	const visibleEntries = await this.getVisibleEntries(filter);
	const windowCenter = Math.round(window.innerHeight / 2);

	let centerMostStop: HTMLElement \| undefined = undefined;
	let minDistanceToCenter = Number.MAX_VALUE;

	for (const entry of visibleEntries) {
	// We are just using the entries here, because entry.boundingClientRect
	// is already computed, but entry.target.getBoundingClientRect() should
	// actually yield the same result.
	const center =
	entry.boundingClientRect.top +
	Math.round(entry.boundingClientRect.height / 2);
	const distanceToWindowCenter = Math.abs(center - windowCenter);
	if (distanceToWindowCenter < minDistanceToCenter) {
	// entry.target comes from the filteredStops array,
	// hence it is an HTMLElement
	centerMostStop = entry.target as HTMLElement;
	minDistanceToCenter = distanceToWindowCenter;
	}
	}
	return centerMostStop;
	}

	private async getVisibleEntries(
	filter?: (el: Element) => boolean
	): Promise<IntersectionObserverEntry[]> {
	if (!this._isIntersectionObserverSupported()) {
	throw new Error('Intersection observing not supported');
	}
	if (!this.stops) {
	return [];
	}
	const filteredStops = filter
	? this.targetableStops.filter(filter)
	: this.targetableStops;
	return new Promise(resolve => {
	let unobservedCount = filteredStops.length;
	const visibleEntries: IntersectionObserverEntry[] = [];
	const observer = new IntersectionObserver(entries => {
	visibleEntries.push(
	...entries
	// In Edge it is recommended to use intersectionRatio instead of
	// isIntersecting.
	.filter(
	entry => entry.isIntersecting \|\| entry.intersectionRatio > 0
	)
	);

	// This callback is called for the first time immediately.
	// Typically it gets all observed stops at once, but
	// sometimes can get them in several chunks.
	for (const entry of entries) {
	observer.unobserve(entry.target);
	}
	unobservedCount -= entries.length;
	if (unobservedCount === 0) {
	resolve(visibleEntries);
	}
	});
	for (const stop of filteredStops) {
	observer.observe(stop);
	}
	});
	}

	_isIntersectionObserverSupported() {
	// The copy of this method exists in gr-app-element.js under the
	// name _isCursorManagerSupportMoveToVisibleLine
	// If you update this method, you must update gr-app-element.js
	// as well.
	return 'IntersectionObserver' in window;
	}

	/**
	* Set the cursor to an arbitrary stop - if the given element is not one of
	* the stops, unset the cursor.
	*
	* @param noScroll prevent any potential scrolling in response
	* setting the cursor.
	*/
	setCursor(element: HTMLElement, noScroll?: boolean) {
	if (!this.targetableStops.includes(element)) {
	this.unsetCursor();
	return;
	}
	let behavior;
	if (noScroll) {
	behavior = this.scrollMode;
	this.scrollMode = ScrollMode.NEVER;
	}

	this.unsetCursor();
	this.target = element;
	this._updateIndex();
	this._decorateTarget();

	if (noScroll && behavior) {
	this.scrollMode = behavior;
	}
	}

	unsetCursor() {
	this._unDecorateTarget();
	this.index = -1;
	this.target = null;
	this._targetHeight = null;
	}

	/** Returns true if there are no stops, or we are on the first stop. */
	isAtStart(): boolean {
	return this.stops.length === 0 \|\| this.index === 0;
	}

	/** Returns true if there are no stops, or we are on the last stop. */
	isAtEnd(): boolean {
	return this.stops.length === 0 \|\| this.index === this.stops.length - 1;
	}

	moveToStart() {
	if (this.stops.length) {
	this.setCursorAtIndex(0);
	}
	}

	moveToEnd() {
	if (this.stops.length) {
	this.setCursorAtIndex(this.stops.length - 1);
	}
	}

	setCursorAtIndex(index: number, noScroll?: boolean) {
	const stop = this.stops[index];
	if (isTargetable(stop)) {
	this.setCursor(stop, noScroll);
	}
	}

	/**
	* Move the cursor forward or backward by delta. Clipped to the beginning or
	* end of stop list.
	*
	* @param delta either -1 or 1.
	* @param options.abort Will abort moving the cursor when encountering a
	* stop for which this condition is met. Will abort even if the stop
	* would have been filtered
	* @param options.filter Will keep going and skip any stops for which this
	* condition is not met.
	* @param options.getTargetHeight Optional function to calculate the
	* height of the target's 'section'. The height of the target itself is
	* sometimes different, used by the diff cursor.
	* @param options.clipToTop When none of the next indices match, move
	* back to first instead of to last.
	* @return If a move was performed or why not.
	* @private
	*/
	_moveCursor(
	delta: number,
	{
	filter,
	getTargetHeight,
	clipToTop,
	}: {
	filter?: (stop: HTMLElement) => boolean;
	getTargetHeight?: (target: HTMLElement) => number;
	clipToTop?: boolean;
	} = {}
	): CursorMoveResult {
	if (!this.stops.length) {
	this.unsetCursor();
	return CursorMoveResult.NO_STOPS;
	}

	let newIndex = this.index;
	// If the cursor is not yet set and we are going backwards, start at the
	// back.
	if (this.index === -1 && delta < 0) {
	newIndex = this.stops.length;
	}

	let clipped = false;
	let newStop: Stop;
	do {
	newIndex += delta;
	if (
	(delta > 0 && newIndex >= this.stops.length) \|\|
	(delta < 0 && newIndex < 0)
	) {
	newIndex = delta < 0 \|\| clipToTop ? 0 : this.stops.length - 1;
	newStop = this.stops[newIndex];
	clipped = true;
	break;
	}
	// Sadly needed so that type narrowing understands that this.stops[newIndex] is
	// targetable after I have checked that.
	newStop = this.stops[newIndex];
	} while (isTargetable(newStop) && filter && !filter(newStop));

	if (!isTargetable(newStop)) {
	return CursorMoveResult.ABORTED;
	}

	this._unDecorateTarget();

	this.index = newIndex;
	this.target = newStop;

	if (getTargetHeight) {
	this._targetHeight = getTargetHeight(this.target);
	} else {
	this._targetHeight = this.target.scrollHeight;
	}

	if (this.focusOnMove) {
	this.target.focus();
	}

	this._decorateTarget();

	return clipped ? CursorMoveResult.CLIPPED : CursorMoveResult.MOVED;
	}

	_decorateTarget() {
	if (this.target && this.cursorTargetClass) {
	this.target.classList.add(this.cursorTargetClass);
	}
	}

	_unDecorateTarget() {
	if (this.target && this.cursorTargetClass) {
	this.target.classList.remove(this.cursorTargetClass);
	}
	}

	_updateIndex() {
	if (!this.target) {
	this.index = -1;
	return;
	}

	const newIndex = this.stops.indexOf(this.target);
	if (newIndex === -1) {
	this.unsetCursor();
	} else {
	this.index = newIndex;
	}
	}

	/**
	* Calculate where the element is relative to the window.
	*
	* @param target Target to scroll to.
	* @return Distance to top of the target.
	*/
	_getTop(target: HTMLElement) {
	let top: number = target.offsetTop;
	for (
	let offsetParent = target.offsetParent;
	offsetParent;
	offsetParent = (offsetParent as HTMLElement).offsetParent
	) {
	top += (offsetParent as HTMLElement).offsetTop;
	}
	return top;
	}

	_targetIsVisible(top: number) {
	return (
	this.scrollMode === ScrollMode.KEEP_VISIBLE &&
	top > window.pageYOffset &&
	top < window.pageYOffset + window.innerHeight
	);
	}

	_calculateScrollToValue(top: number, target: HTMLElement) {
	return top + -window.innerHeight / 3 + target.offsetHeight / 2;
	}

	_scrollToTarget() {
	if (!this.target \|\| this.scrollMode === ScrollMode.NEVER) {
	return;
	}

	const top = this._getTop(this.target);
	const bottomIsVisible = this._targetHeight
	? this._targetIsVisible(top + this._targetHeight)
	: true;
	const scrollToValue = this._calculateScrollToValue(top, this.target);

	if (this._targetIsVisible(top)) {
	// Don't scroll if either the bottom is visible or if the position that
	// would get scrolled to is higher up than the current position. This
	// would cause less of the target content to be displayed than is
	// already.
	if (bottomIsVisible \|\| scrollToValue < window.scrollY) {
	return;
	}
	}

	// Scroll the element to the middle of the window. Dividing by a third
	// instead of half the inner height feels a bit better otherwise the
	// element appears to be below the center of the window even when it
	// isn't.
	window.scrollTo(window.scrollX, scrollToValue);
	}
	}