New Twiddle

controllers.application\.js

import Controller from '@ember/controller';
import { tracked } from '@glimmer/tracking';
import { action } from '@ember/object';

export default class Application extends Controller {
  @tracked size = {
    height: 'unknown',
    width: 'unknown',
  };

  @tracked runCount = 0;

  @action onResize({ width, element, runCount }) {
    this.size = {
      height: element.clientHeight,
      width: width,
    };
    this.runCount = runCount;
  }
}

modifiers.resize-observer\.js

import { modifier } from 'ember-modifier';

export function resizeObserver(
  element,
  [onResize],
  { hasResizeObserver = 'ResizeObserver' in window }
) {
  // Note: The resize observer will also run initially
  // If it is not supported, we only call it once, 
  // but it will not trigger on actual resizing
  // Note: This can be overwritten for tests
  if (!hasResizeObserver) {
    let width = Math.floor(element.offsetWidth);
    onResize({ width, element, runCount: 0 });
    return;
  }

  let runCount = 0;
  let previousWidth;
  let resizeObserver = new ResizeObserver(() => {
    let width = Math.floor(element.offsetWidth);

    if (width === previousWidth) {
      return;
    }

    onResize({ width, element, runCount });
    runCount++;
    previousWidth = width;
  });

  resizeObserver.observe(element);

  return () => resizeObserver.disconnect();
}

export default modifier(resizeObserver);

templates.application\.hbs

<h1>Modifier in Safari repro</h1>

<ul>
  <li>Run count: {{this.runCount}}</li>
  <li>Width: {{this.size.width}}</li>
  <li>Height: {{this.size.height}}</li>
</ul>

<div {{resize-observer this.onResize}}>
	Check resize on this element...
</div>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0",
    "ember-modifier": "1.0.3"
  }
}