>Is there a better solution

Yes. And it's pretty simple:

    WebDriver driver = new FirefoxDriver();
    driver.get("http://somedomain/url_that_delays_loading");
    WebElement myDynamicElement = (new WebDriverWait(driver, 10))
  .until(ExpectedConditions.presenceOfElementLocated(By.id("myDynamicElement")));

From : http://docs.seleniumhq.org/docs/04_webdriver_advanced.jsp

I'm not sure this satisfies your parent poster's requirement of: "if it fails, you know about it pretty soon, without the need for long timeouts."

Well, you need to have a timeout.

You can make the timeout shorter when running the test on a dev environment, though, so you get quicker feedback about errors.

Ruby's Capybara encapsulates Selenium and waits until elements appear on the page (the default timeout is 2 seconds). So you can write simple sequential code like

    click_link('bar')
    expect(page).to have_content('baz')

and it will work even if the baz element is injected into the page by an Ajax request to the server triggered by clicking on bar. I've been using it for many years but I didn't check how they implement it. Maybe a callback from a MutationObserver? https://developer.mozilla.org/en-US/docs/Web/API/MutationObs...

Documentation at https://github.com/jnicklas/capybara#asynchronous-javascript...

According to that documentation you linked, it just polls until `default_max_wait_time` (which defaults to 2 seconds).

I have had some good results using the F# canopy library(http://lefthandedgoat.github.io/canopy/) for working with selenium. It handles (most) all the waits for you so you don't have to scatter a bunch of sleeps in your tests and is pretty easy to work with.

There's some utility methods like FluentWait, but ultimately they're just convenience wrappers for polling the DOM and waiting.