alexreardon

Generated by AI

CSS Anchor Positioning: Auto-Flipping Arrows with Popovers

Purpose: This document is a comprehensive reference for developers and AI assistants implementing CSS anchor-positioned popovers with arrows that automatically flip direction when using position-try-fallbacks. It covers the core technique, the position-area property (including the critical span keywords), and complete working examples.

Table of Contents

1. The Problem
2. The Solution: clip-path margin-box

alexreardon

// A simple script to clear all notifications in Slack.
// 1. Open Slack on the web (not in the app as you cannot open the dev tools console)
// 2. Copy paste into your console
(async () => {
  function goToNotifications() {
    const button = document.querySelector('[data-qa="tab_rail_activity_button"]');
    if (!(button instanceof HTMLElement)) {
      throw Error('Count not find activities button');
    }
    button.click();

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// This file polyfills DOMRect
// DOMRect is currently not polyfilled by jsdom

(() => {
	if (typeof window === 'undefined') {
		return;
	}
	if (window.DOMRect) {
		return;
	}

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/* data-transfer-item-list.spec.ts */

import invariant from 'tiny-invariant';

test('add(data, format) should add a string item', done => {
  const list = new DataTransferItemList();

  list.add('Hello world', 'text/plain');

  const item: DataTransferItem = list[0];

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// This file polyfills DragEvent for jsdom
// https://github.com/jsdom/jsdom/issues/2913
// This file is in JS rather than TS, as our jsdom setup files currently need to be in JS
// Good news: DragEvents are almost the same as MouseEvents

(() => {
  if (typeof window === 'undefined') {
    return;
  }

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function add(a: number, b: number): number {
  return a + b;
}

type MyParameters<TFunc extends (...args: any[]) => unknown> = TFunc extends (
  ...args: infer TArgs
) => unknown
  ? TArgs
  : unknown;

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// This file polyfills `Element.prototype.scrollBy`
// scrollBy(x-coord, y-coord)
// scrollBy(options)
(() => {
  if (typeof Element === 'undefined') {
    return;
  }
  if (typeof Element.prototype.scrollBy !== 'undefined') {
    return;
  }

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const interactions: number[][] = [];

function sum(values: number[]) {
  return values.reduce((acc, current) => acc + current, 0);
}

function average(values: number[]): number {
  if (!values.length) {
    return 0;
  }

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export {};

const allCollectedFps: number[][] = [];

function sum(values: number[]) {
  return values.reduce((acc, current) => acc + current, 0);
}

function average(values: number[]): number {
  if (!values.length) {

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function average(values: number[]): number {
  const sum = values.reduce((acc, current) => acc + current, 0);
  return sum / values.length;
}

export function standardDeviation(values: number[]): number {
  /** https://www.mathsisfun.com/data/standard-deviation-formulas.html
   * 1. Work out the Mean (the simple average of the numbers)
   * 2. Then for each number: subtract the Mean and square the result
   * 3. Then work out the mean of those squared differences.