[BUG] firefox's `node.scrollIntoView({behavior: 'instant'})` is not instant :(

[aslushnikov]

This is investigation for the failing firefox test:

playwright/test/elementhandle.spec.js

Line 303 in b24262b

it.fail(FFOX)('should work', async({page, server}) => {

1. Looks like Firefox doesn't support 'instant' behavior in scrollIntoView() argument.
2. It was noticed before: https://bugzilla.mozilla.org/show_bug.cgi?id=1570605

The following snippet demonstrates that window.scrollTo(0, 0) doesn't actually scroll to (0, 0).. And the element.scrollIntoView() command continues working after we scroll window.

const {firefox, chromium} = require('.');

(async() => {
  const browser = await firefox.launch();
  const context = await browser.newContext();
  const page = await context.newPage();
  await page.setContent(`
    <style>
      * {
        margin: 0;
        padding: 0;
        position: absolute;
      }

      body, html {
        width: 100%;
        height: 100%;
      }

      button {
        width: 100px;
        height: 20px;
        right: -20px;
        top: 50px;
      }
    </style>
    <button>yo</button>
  `);

  console.log(await page.evaluate(async () => {
    const measures = [];
    document.querySelector('button').scrollIntoView({behavior: 'auto'});
    measures.push({
      'window.scrollX': window.scrollX,
      'window.scrollY': window.scrollY,
    });
    await new Promise(x => setTimeout(x, 30));
    window.scrollTo(0, 0);
    measures.push({
      'window.scrollX': window.scrollX,
      'window.scrollY': window.scrollY,
    });
    for (let i = 0; i < 15; ++i) {
      await new Promise(x => requestAnimationFrame(x));
      //await new Promise(x => setTimeout(x, 30));
      measures.push({
        'window.scrollX': window.scrollX,
        'window.scrollY': window.scrollY,
      });
    }
    return measures;
  }));
  await browser.close();
})();

Compare output results between Chromium and Firefox browsers. On Chromium, all measurements after window.scrollTo(0, 0) end up to be (0, 0). On Firefox, they return to (20, 0) for me.

[pavelfeldman]

I think Yury foxed it