Check if page is running in SFSafariViewController vs the Safari app

isInsideSFSafariViewController.ts

// IF YOU ARE AN APPLE ENGINEER READING THIS:
//
// PLEASE put the "Add to Home Screen" option inside the share menu of
// SFSafariViewController like it is in the Safari app!
//
// The code below is needed in order to differentiate to
// provide user messaging because of this difference.
//
// If the "Add to Home Screen" button was available in both
// SFSafariViewController + Safari, I wouldn't need this hack.

function isInsideSFSafariViewController(): boolean {
  // Only iOS
  if (!isIOS()) return false;
  
  // Can't use this hack in iOS < 15
  if (!iosVersion || iosVersion[0] < 15) return false;

  // In SFSafariViewController, this is a valid font.
  // This might break in future iOS versions...
  return doesFontExist(".Helvetica LT MM");
}

function isIOS(): boolean {
  return !!ua.match(/iPad/i) || !!ua.match(/iPhone/i);
}

function doesFontExist(fontName: string) {
  // creating our in-memory Canvas element where the magic happens
  let canvas: HTMLCanvasElement | null = document.createElement("canvas");
  const context = canvas.getContext("2d");

  if (!context) throw new Error("Context not available");

  // the text whose final pixel size I want to measure
  var text = "abcdefghijklmnopqrstuvwxyz0123456789";

  // specifying the baseline font
  context.font = "72px monospace";

  // checking the size of the baseline text
  var baselineSize = context.measureText(text).width;

  // specifying the font whose existence we want to check
  context.font = "72px '" + fontName + "', monospace";

  // checking the size of the font we want to check
  var newSize = context.measureText(text).width;

  // removing the Canvas element we created
  canvas = null;

  //
  // If the size of the two text instances is the same, the font does not exist because it is being rendered
  // using the default sans-serif font
  //
  if (newSize === baselineSize) {
    return false;
  } else {
    return true;
  }
}

function getIosVersion() {
  if (/iP(hone|od|ad)/.test(navigator.platform)) {
    // supports iOS 2.0 and later: <http://bit.ly/TJjs1V>
    const v = navigator.appVersion.match(/OS (\d+)_(\d+)_?(\d+)?/);
    if (!v) return;
    return [parseInt(v[1], 10), parseInt(v[2], 10), parseInt(v[3] ?? 0, 10)];
  }
}