fune/browser/base/content/test/general/browser_visibleFindSelection.js

add_task(async function() {
  const childContent = "<div style='position: absolute; left: 2200px; background: green; width: 200px; height: 200px;'>" +
                       "div</div><div  style='position: absolute; left: 0px; background: red; width: 200px; height: 200px;'>" +
                       "<span id='s'>div</span></div>";

  let tab = gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);

  await promiseTabLoadEvent(tab, "data:text/html;charset=utf-8," + escape(childContent));
  await SimpleTest.promiseFocus(gBrowser.selectedBrowser);

  let remote = gBrowser.selectedBrowser.isRemoteBrowser;

  let findBarOpenPromise = promiseWaitForEvent(gBrowser, "findbaropen");
  EventUtils.synthesizeKey("f", {accelKey: true});
  await findBarOpenPromise;

  ok(gFindBarInitialized, "find bar is now initialized");

  // Finds the div in the green box.
  let scrollPromise = remote ?
    BrowserTestUtils.waitForContentEvent(gBrowser.selectedBrowser, "scroll") :
    BrowserTestUtils.waitForEvent(gBrowser, "scroll");
  EventUtils.sendString("div");
  await scrollPromise;

  // Wait for one paint to ensure we've processed the previous key events and scrolling.
  await ContentTask.spawn(gBrowser.selectedBrowser, null, async function() {
    return new Promise(
      resolve => {
        content.requestAnimationFrame(() => {
          setTimeout(resolve, 0);
        });
      }
    );
  });

  // Finds the div in the red box.
  scrollPromise = remote ?
    BrowserTestUtils.waitForContentEvent(gBrowser.selectedBrowser, "scroll") :
    BrowserTestUtils.waitForEvent(gBrowser, "scroll");
  EventUtils.synthesizeKey("g", {accelKey: true});
  await scrollPromise;

  await ContentTask.spawn(gBrowser.selectedBrowser, null, async function() {
    Assert.ok(content.document.getElementById("s").getBoundingClientRect().left >= 0,
      "scroll should include find result");
  });

  // clear the find bar
  EventUtils.synthesizeKey("a", {accelKey: true});
  EventUtils.synthesizeKey("KEY_Delete");

  gFindBar.close();
  gBrowser.removeCurrentTab();
});