/
font_preferences.ts
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/
font_preferences.ts
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import { isChromium, isWebKit } from '../utils/browser'
import { withIframe } from '../utils/dom'
import { MaybePromise } from '../utils/async'
type WritableCSSProperties = {
[K in keyof CSSStyleDeclaration]: CSSStyleDeclaration[K] extends string ? K : never
}[Extract<keyof CSSStyleDeclaration, string>]
type WritableCSSStyles = Partial<Pick<CSSStyleDeclaration, WritableCSSProperties>>
type Preset = [style?: WritableCSSStyles, text?: string]
/**
* We use m or w because these two characters take up the maximum width.
* Also there are a couple of ligatures.
*/
const defaultText = 'mmMwWLliI0fiflO&1'
/**
* Settings of text blocks to measure. The keys are random but persistent words.
*/
export const presets: Record<string, Preset> = {
/**
* The default font. User can change it in desktop Chrome, desktop Firefox, IE 11,
* Android Chrome (but only when the size is ≥ than the default) and Android Firefox.
*/
default: [],
/** OS font on macOS. User can change its size and weight. Applies after Safari restart. */
apple: [{ font: '-apple-system-body' }],
/** User can change it in desktop Chrome and desktop Firefox. */
serif: [{ fontFamily: 'serif' }],
/** User can change it in desktop Chrome and desktop Firefox. */
sans: [{ fontFamily: 'sans-serif' }],
/** User can change it in desktop Chrome and desktop Firefox. */
mono: [{ fontFamily: 'monospace' }],
/**
* Check the smallest allowed font size. User can change it in desktop Chrome, desktop Firefox and desktop Safari.
* The height can be 0 in Chrome on a retina display.
*/
min: [{ fontSize: '1px' }],
/** Tells one OS from another in desktop Chrome. */
system: [{ fontFamily: 'system-ui' }],
}
/**
* The result is a dictionary of the width of the text samples.
* Heights aren't included because they give no extra entropy and are unstable.
*
* The result is very stable in IE 11, Edge 18 and Safari 14.
* The result changes when the OS pixel density changes in Chromium 87. The real pixel density is required to solve,
* but seems like it's impossible: https://stackoverflow.com/q/1713771/1118709.
* The "min" and the "mono" (only on Windows) value may change when the page is zoomed in Firefox 87.
*/
export default function getFontPreferences(): Promise<Record<string, number>> {
return withNaturalFonts((document, container) => {
const elements: Record<string, HTMLElement> = {}
const sizes: Record<string, number> = {}
// First create all elements to measure. If the DOM steps below are done in a single cycle,
// browser will alternate tree modification and layout reading, that is very slow.
for (const key of Object.keys(presets)) {
const [style = {}, text = defaultText] = presets[key]
const element = document.createElement('span')
element.textContent = text
element.style.whiteSpace = 'nowrap'
for (const name of Object.keys(style) as Array<keyof typeof style>) {
const value = style[name]
if (value !== undefined) {
element.style[name] = value
}
}
elements[key] = element
container.append(document.createElement('br'), element)
}
// Then measure the created elements
for (const key of Object.keys(presets)) {
sizes[key] = elements[key].getBoundingClientRect().width
}
return sizes
})
}
/**
* Creates a DOM environment that provides the most natural font available, including Android OS font.
* Measurements of the elements are zoom-independent.
* Don't put a content to measure inside an absolutely positioned element.
*/
function withNaturalFonts<T>(
action: (document: Document, container: HTMLElement) => MaybePromise<T>,
containerWidthPx = 4000,
): Promise<T> {
/*
* Requirements for Android Chrome to apply the system font size to a text inside an iframe:
* - The iframe mustn't have a `display: none;` style;
* - The text mustn't be positioned absolutely;
* - The text block must be wide enough.
* 2560px on some devices in portrait orientation for the biggest font size option (32px);
* - There must be much enough text to form a few lines (I don't know the exact numbers);
* - The text must have the `text-size-adjust: none` style. Otherwise the text will scale in "Desktop site" mode;
*
* Requirements for Android Firefox to apply the system font size to a text inside an iframe:
* - The iframe document must have a header: `<meta name="viewport" content="width=device-width, initial-scale=1" />`.
* The only way to set it is to use the `srcdoc` attribute of the iframe;
* - The iframe content must get loaded before adding extra content with JavaScript;
*
* https://example.com as the iframe target always inherits Android font settings so it can be used as a reference.
*
* Observations on how page zoom affects the measurements:
* - macOS Safari 11.1, 12.1, 13.1, 14.0: zoom reset + offsetWidth = 100% reliable;
* - macOS Safari 11.1, 12.1, 13.1, 14.0: zoom reset + getBoundingClientRect = 100% reliable;
* - macOS Safari 14.0: offsetWidth = 5% fluctuation;
* - macOS Safari 14.0: getBoundingClientRect = 5% fluctuation;
* - iOS Safari 9, 10, 11.0, 12.0: haven't found a way to zoom a page (pinch doesn't change layout);
* - iOS Safari 13.1, 14.0: zoom reset + offsetWidth = 100% reliable;
* - iOS Safari 13.1, 14.0: zoom reset + getBoundingClientRect = 100% reliable;
* - iOS Safari 14.0: offsetWidth = 100% reliable;
* - iOS Safari 14.0: getBoundingClientRect = 100% reliable;
* - Chrome 42, 65, 80, 87: zoom 1/devicePixelRatio + offsetWidth = 1px fluctuation;
* - Chrome 42, 65, 80, 87: zoom 1/devicePixelRatio + getBoundingClientRect = 100% reliable;
* - Chrome 87: offsetWidth = 1px fluctuation;
* - Chrome 87: getBoundingClientRect = 0.7px fluctuation;
* - Firefox 48, 51: offsetWidth = 10% fluctuation;
* - Firefox 48, 51: getBoundingClientRect = 10% fluctuation;
* - Firefox 52, 53, 57, 62, 66, 67, 68, 71, 75, 80, 84: offsetWidth = width 100% reliable, height 10% fluctuation;
* - Firefox 52, 53, 57, 62, 66, 67, 68, 71, 75, 80, 84: getBoundingClientRect = width 100% reliable, height 10%
* fluctuation;
* - Android Chrome 86: haven't found a way to zoom a page (pinch doesn't change layout);
* - Android Firefox 84: font size in accessibility settings changes all the CSS sizes, but offsetWidth and
* getBoundingClientRect keep measuring with regular units, so the size reflects the font size setting and doesn't
* fluctuate;
* - IE 11, Edge 18: zoom 1/devicePixelRatio + offsetWidth = 100% reliable;
* - IE 11, Edge 18: zoom 1/devicePixelRatio + getBoundingClientRect = reflects the zoom level;
* - IE 11, Edge 18: offsetWidth = 100% reliable;
* - IE 11, Edge 18: getBoundingClientRect = 100% reliable;
*/
return withIframe((_, iframeWindow) => {
const iframeDocument = iframeWindow.document
const iframeBody = iframeDocument.body
const bodyStyle = iframeBody.style
bodyStyle.width = `${containerWidthPx}px`
bodyStyle.webkitTextSizeAdjust = bodyStyle.textSizeAdjust = 'none'
// See the big comment above
if (isChromium()) {
iframeBody.style.zoom = `${1 / iframeWindow.devicePixelRatio}`
} else if (isWebKit()) {
iframeBody.style.zoom = 'reset'
}
// See the big comment above
const linesOfText = iframeDocument.createElement('div')
linesOfText.textContent = [...Array((containerWidthPx / 20) << 0)].map(() => 'word').join(' ')
iframeBody.appendChild(linesOfText)
return action(iframeDocument, iframeBody)
}, '<!doctype html><html><head><meta name="viewport" content="width=device-width, initial-scale=1">')
}