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Reset scroll tracking across document resizes #525

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66 changes: 57 additions & 9 deletions src/interactionMonitor.ts
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Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
import type {EventListener} from './eventManager';
import {SynchronousEventManager} from './eventManager';
import type {UserClicked, UserScrolled, Point} from './trackingEvents';
import type {UserClicked, UserScrolled, Point, Size} from './trackingEvents';

type InteractionEventMap = {
userClicked: UserClicked,
Expand All @@ -15,6 +15,7 @@ type Options = {
type ScrollState = {
start: Point,
lastPosition: Point,
surfaceSize: Size,
};

export class InteractionMonitor {
Expand All @@ -38,6 +39,7 @@ export class InteractionMonitor {

this.handleClick = this.handleClick.bind(this);
this.handleScroll = this.handleScroll.bind(this);
this.handleResize = this.handleResize.bind(this);
}

public addListener<T extends keyof InteractionEventMap>(
Expand Down Expand Up @@ -67,6 +69,7 @@ export class InteractionMonitor {

window.addEventListener('click', this.handleClick, true);
window.addEventListener('scroll', this.handleScroll, true);
window.addEventListener('resize', this.handleResize, true);
}

public disable(): void {
Expand All @@ -78,6 +81,7 @@ export class InteractionMonitor {

window.removeEventListener('click', this.handleClick, true);
window.removeEventListener('scroll', this.handleScroll, true);
window.removeEventListener('resize', this.handleResize, true);

this.flushPendingScroll();
}
Expand All @@ -104,16 +108,48 @@ export class InteractionMonitor {
});
}

private handleResize(): void {
if (this.scrollDebounceTimer !== undefined) {
window.clearTimeout(this.scrollDebounceTimer);
this.scrollDebounceTimer = undefined;
}

const currentPosition: Point = {
x: Math.max(0, Math.round(window.scrollX)),
y: Math.max(0, Math.round(window.scrollY)),
};

const currentSurfaceSize: Size = {
width: document.documentElement.scrollWidth,
height: document.documentElement.scrollHeight,
};

this.scrollState = {
start: currentPosition,
lastPosition: currentPosition,
surfaceSize: currentSurfaceSize,
};
}

private handleScroll(): void {
const currentPosition: Point = {
x: Math.max(0, Math.round(window.scrollX)),
y: Math.max(0, Math.round(window.scrollY)),
};

if (this.scrollState === undefined) {
const currentSurfaceSize: Size = {
width: document.documentElement.scrollWidth,
height: document.documentElement.scrollHeight,
};

if (
this.scrollState?.surfaceSize.width !== currentSurfaceSize.width
|| this.scrollState.surfaceSize.height !== currentSurfaceSize.height
) {
this.scrollState = {
start: currentPosition,
lastPosition: currentPosition,
surfaceSize: currentSurfaceSize,
};
} else if (this.hasDirectionChanged(this.scrollState, currentPosition)) {
const turningPoint = this.scrollState.lastPosition;
Expand All @@ -123,6 +159,7 @@ export class InteractionMonitor {
this.scrollState = {
start: turningPoint,
lastPosition: currentPosition,
surfaceSize: currentSurfaceSize,
};
} else {
this.scrollState.lastPosition = currentPosition;
Expand Down Expand Up @@ -190,14 +227,28 @@ export class InteractionMonitor {
return;
}

const {start} = this.scrollState;
const state = this.scrollState;

this.scrollState = undefined;

const currentSurfaceSize: Size = {
width: document.documentElement.scrollWidth,
height: document.documentElement.scrollHeight,
};

if (
state.surfaceSize.width !== currentSurfaceSize.width
|| state.surfaceSize.height !== currentSurfaceSize.height
) {
return;
}

const {start} = state;
const destination = end ?? {
x: Math.max(0, Math.round(window.scrollX)),
y: Math.max(0, Math.round(window.scrollY)),
};

this.scrollState = undefined;

if (start.x === destination.x && start.y === destination.y) {
return;
}
Expand All @@ -206,10 +257,7 @@ export class InteractionMonitor {
type: 'userScrolled',
start: start,
end: destination,
surfaceSize: {
width: document.documentElement.scrollWidth,
height: document.documentElement.scrollHeight,
},
surfaceSize: state.surfaceSize,
// Uses clientWidth/clientHeight instead of innerWidth/innerHeight to get the
// layout viewport size, which remains stable regardless of pinch-to-zoom level
// on mobile devices.
Expand Down
133 changes: 133 additions & 0 deletions test/interactionMonitor.test.ts
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -4,13 +4,30 @@ import type {UserClicked, UserScrolled} from '../src/trackingEvents';

describe('An interaction monitor', () => {
const tabEventEmulator = new TabEventEmulator();
let originalScrollWidth: PropertyDescriptor | undefined;
let originalScrollHeight: PropertyDescriptor | undefined;

beforeEach(() => {
originalScrollWidth = Object.getOwnPropertyDescriptor(document.documentElement, 'scrollWidth');
originalScrollHeight = Object.getOwnPropertyDescriptor(document.documentElement, 'scrollHeight');

jest.useFakeTimers();
tabEventEmulator.registerListeners();
});

afterEach(() => {
if (originalScrollWidth === undefined) {
Reflect.deleteProperty(document.documentElement, 'scrollWidth');
} else {
Object.defineProperty(document.documentElement, 'scrollWidth', originalScrollWidth);
}

if (originalScrollHeight === undefined) {
Reflect.deleteProperty(document.documentElement, 'scrollHeight');
} else {
Object.defineProperty(document.documentElement, 'scrollHeight', originalScrollHeight);
}

jest.clearAllMocks();
jest.useRealTimers();
tabEventEmulator.reset();
Expand All @@ -35,6 +52,13 @@ describe('An interaction monitor', () => {
tabEventEmulator.dispatchEvent(window, new Event('scroll', {bubbles: true}));
}

function setDocumentDimensions(width: number, height: number): void {
Object.defineProperties(document.documentElement, {
scrollWidth: {configurable: true, value: width},
scrollHeight: {configurable: true, value: height},
});
}

it('should track a click with correct position and surface size', () => {
const listener = jest.fn();
const monitor = new InteractionMonitor();
Expand Down Expand Up @@ -205,6 +229,86 @@ describe('An interaction monitor', () => {
monitor.disable();
});

it('should discard a pending scroll after document dimensions change', () => {
const listener = jest.fn();
const monitor = new InteractionMonitor({scrollDebounceInterval: 150});

setDocumentDimensions(1000, 5000);
monitor.addListener('userScrolled', listener);
monitor.enable();

scrollTo(0, 4000);
scrollTo(0, 4500);

setDocumentDimensions(1000, 800);
scrollTo(0, 0);
scrollTo(0, 200);

jest.advanceTimersByTime(150);

expect(listener).toHaveBeenCalledTimes(1);

const event = listener.mock.calls[0][0] as UserScrolled;

expect(event.start).toEqual({x: 0, y: 0});
expect(event.end).toEqual({x: 0, y: 200});
expect(event.surfaceSize).toEqual({width: 1000, height: 800});

monitor.disable();
});

it('should suppress a pending scroll when dimensions change before debounce flush', () => {
const listener = jest.fn();
const monitor = new InteractionMonitor({scrollDebounceInterval: 150});

setDocumentDimensions(1000, 5000);
monitor.addListener('userScrolled', listener);
monitor.enable();

scrollTo(0, 4000);
scrollTo(0, 4500);

setDocumentDimensions(1000, 800);

jest.advanceTimersByTime(150);

expect(listener).not.toHaveBeenCalled();

monitor.disable();
});

it('should discard a pending scroll and establish a new baseline on resize', () => {
const listener = jest.fn();
const monitor = new InteractionMonitor({scrollDebounceInterval: 150});

setDocumentDimensions(1000, 5000);
monitor.addListener('userScrolled', listener);
monitor.enable();

scrollTo(50, 800);
scrollTo(150, 1600);

Object.defineProperties(window, {
scrollX: {configurable: true, value: -10.6},
scrollY: {configurable: true, value: 200.6},
});
setDocumentDimensions(800, 1200);
tabEventEmulator.dispatchEvent(window, new Event('resize'));

scrollTo(300.2, 400.8);
jest.advanceTimersByTime(150);

expect(listener).toHaveBeenCalledTimes(1);

const event = listener.mock.calls[0][0] as UserScrolled;

expect(event.start).toEqual({x: 0, y: 201});
expect(event.end).toEqual({x: 300, y: 401});
expect(event.surfaceSize).toEqual({width: 800, height: 1200});

monitor.disable();
});

it('should collapse continuous scrolling into a single event', () => {
const listener = jest.fn();
const monitor = new InteractionMonitor({scrollDebounceInterval: 150});
Expand Down Expand Up @@ -370,6 +474,35 @@ describe('An interaction monitor', () => {
expect(event.end).toEqual({x: 150, y: 300});
});

it('should not establish a scroll baseline from a resize after disable', () => {
const listener = jest.fn();
const monitor = new InteractionMonitor({scrollDebounceInterval: 150});

tabEventEmulator.reset();
monitor.addListener('userScrolled', listener);
monitor.enable();
monitor.disable();

Object.defineProperties(window, {
scrollX: {configurable: true, value: 100},
scrollY: {configurable: true, value: 200},
});
window.dispatchEvent(new Event('resize'));

monitor.enable();

Object.defineProperties(window, {
scrollX: {configurable: true, value: 300},
scrollY: {configurable: true, value: 400},
});
window.dispatchEvent(new Event('scroll'));
jest.advanceTimersByTime(150);

expect(listener).not.toHaveBeenCalled();

monitor.disable();
});

it('should be idempotent on enable', () => {
const listener = jest.fn();
const monitor = new InteractionMonitor();
Expand Down
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