Boost for Arc Browser (https://arc.net/) based on this tweet: https://twitter.com/adithyashreshti/status/1579896377366044672

arc-whatsapp-sidebar.js

const HALF_OPEN_WIDTH = 80
// this value starts as -1
// and is updated when the sidebar is updated the first time
// this is the width to which the sidebar will resize when it is fully open
let SIDEBAR_WIDTH = -1

// state enumerator, can only be closed, half-closed, and open
const state = {
	closed: 0,
	halfClosed: 1,
	open: 2,
}

// initial state of the sidebar is open
let currentState = state.open

// waits for the sidebar to open to start the script
const waitForElement = (selector) => {
	return new Promise((resolve) => {
		const elementToLookup = document.querySelector(selector)
		if (elementToLookup) {
			resolve(elementToLookup)
		}
		new MutationObserver((_, observer) => {
			// Query for elements matching the specified selector
			Array.from(document.querySelectorAll(selector)).forEach((element) => {
				resolve(element)
				//Once we have resolved we don't need the observer anymore.
				observer.disconnect()
			})
		}).observe(document.documentElement, {
			childList: true,
			subtree: true,
		})
	})
}

function createExpandButton(element) {
	const btn = document.createElement('button')
	btn.innerHTML =
		'<svg width="24px" height="24px" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><path d="M15.2928932,12 L12.1464466,8.85355339 C11.9511845,8.65829124 11.9511845,8.34170876 12.1464466,8.14644661 C12.3417088,7.95118446 12.6582912,7.95118446 12.8535534,8.14644661 L16.8535534,12.1464466 C17.0488155,12.3417088 17.0488155,12.6582912 16.8535534,12.8535534 L12.8535534,16.8535534 C12.6582912,17.0488155 12.3417088,17.0488155 12.1464466,16.8535534 C11.9511845,16.6582912 11.9511845,16.3417088 12.1464466,16.1464466 L15.2928932,13 L4.5,13 C4.22385763,13 4,12.7761424 4,12.5 C4,12.2238576 4.22385763,12 4.5,12 L15.2928932,12 Z M19,5.5 C19,5.22385763 19.2238576,5 19.5,5 C19.7761424,5 20,5.22385763 20,5.5 L20,19.5 C20,19.7761424 19.7761424,20 19.5,20 C19.2238576,20 19,19.7761424 19,19.5 L19,5.5 Z"/></svg>'

	Object.assign(btn.style, {
		position: 'absolute',
		top: '0',
		left: '0',
		margin: '50px 0',
		padding: '15px 15px 15px 5px',
		zIndex: '9999',
		borderRadius: '0 50px 50px 0',
		backgroundColor: '#00a884',
		boxShadow: '10px 20px 52px 3px #037259',
		transitionDuration: '0.3s',
	})

	// add little expand animation
	btn.addEventListener('mouseenter', () => {
		btn.style.transform = 'scale(1.1)'
	})
	btn.addEventListener('mouseleave', () => {
		btn.style.transform = 'scale(1)'
	})

	// when click on the button, show element
	btn.addEventListener('click', () => {
		// if it is closed, make it half open
		if (currentState === state.closed) {
			element.style.display = 'block'
			currentState = state.halfClosed
			setSideBarWidth(HALF_OPEN_WIDTH)
		} else if (currentState === state.halfClosed) {
			// if it is half open, make it fully open
			currentState = state.open
			setSideBarWidth(SIDEBAR_WIDTH)
			btn.remove()

			// make all divs with this data-testid "icon-unread-count" move left
			// so that they are not hidden by the sidebar
			const unreadCountDivs = document.querySelectorAll('[data-testid="icon-unread-count"]')
			unreadCountDivs.forEach((div) => {
				// move right 37px
				div.style.left = '15px'
			})

			// show the div with the _1EUay class
			const div = document.querySelector("div[class='_1EUay']")
      div.style.display = 'block'
		}
	})

	return btn
}

function createHideButton(element, expandBtn) {
	const btn = document.createElement('button')
	btn.innerHTML =
		'<svg fill="#54656F" width="17px" heigth="17px" viewBox="-32 0 512 512" xmlns="http://www.w3.org/2000/svg"><path d="M257.5 445.1l-22.2 22.2c-9.4 9.4-24.6 9.4-33.9 0L7 273c-9.4-9.4-9.4-24.6 0-33.9L201.4 44.7c9.4-9.4 24.6-9.4 33.9 0l22.2 22.2c9.5 9.5 9.3 25-.4 34.3L136.6 216H424c13.3 0 24 10.7 24 24v32c0 13.3-10.7 24-24 24H136.6l120.5 114.8c9.8 9.3 10 24.8.4 34.3z"/></svg>'
	btn.style.padding = '0 15px'
	btn.addEventListener('click', () => {
		if (currentState === state.open) {
			// if side bar is open, half closes it
			currentState = state.halfClosed
			setSideBarWidth(HALF_OPEN_WIDTH)

			// make all divs with this data-testid "icon-unread-count" move left
			// so that they are not hidden by the sidebar
			const unreadCountDivs = document.querySelectorAll('[data-testid="icon-unread-count"]')
			unreadCountDivs.forEach((div) => {
				// change position to relative
				div.style.position = 'relative'
				// move left 10 px
				div.style.left = '-37px'
			})

			// hide the div with the _1EUay class
      const div = document.querySelector("div[class='_1EUay']")
      div.style.display = 'none'

		} else if (currentState === state.halfClosed) {
			// if side bar is half open, closes it completely
			currentState = state.closed
			element.style.display = 'none'
		}

		if (!expandBtn.parentElement)
			// create expand button
			document.body.appendChild(expandBtn)
	})

	return btn
}

waitForElement("div[class='_2Ts6i _3RGKj']").then((element) => {
	header = element.querySelector('header')
	// add button to header
	expandBtn = createExpandButton(element)
	header.appendChild(createHideButton(element, expandBtn))
})

function setSideBarWidth(width) {
	const sidebar = document.querySelector("div[class='_2Ts6i _3RGKj']")
	const mainContent = document.querySelector("div[class='_2Ts6i _2xAQV']")

	let sidebarWidth = sidebar.clientWidth
	if (SIDEBAR_WIDTH === -1) SIDEBAR_WIDTH = sidebarWidth
	let mainContentWidth = mainContent.clientWidth

	// how much sidebar has to change to get to width
	const sidebarWidthDiff = sidebarWidth - width

	// resize sidebar and main content
	sidebarWidth = width
	mainContentWidth = mainContentWidth + sidebarWidthDiff

	// set important css for the width
	sidebar.style.width = sidebarWidth + 'px'
	sidebar.style.flex = '0 0 ' + sidebarWidth + 'px'
	mainContent.style.width = mainContentWidth + 'px'
	mainContent.style.flex = '0 0 ' + mainContentWidth + 'px'
}