I am testing a feature that uses an scroll event listener. I am using the event listener inside a JS hook.

Basically this.

mounted() {
  this.el.addEventListener('scroll', () => {
    ...
    this.pushEvent("scroll_changed", ...)
  })
}

I am using Wallaby to test this feature, and I have set up Wallaby to scroll to a certain location during the test. And using take_screenshot() I have confirmed the page indeed gets scrolled to the right location.

feature "", %{session, session} do
  user = ...

  session
  |> log_in_user(user)
  |> visit(~p"/read/#{story.id}")
  |> scroll_into_view(@resume_location)
end

def scroll_into_view(session, selector) do
  execute_script(
    session,
    """
    document.querySelector(arguments[0]).scrollIntoView({
      behavior: 'auto',
      block: 'center',
      inline: 'center'
    })
    """,
    [selector]
  )
end

I expect to handle the “scroll_changed” event during testing. Outside of testing I always receive the event. During testing I don’t receive the event.

This IO.inspect() never gets printed.

def handle_event("scroll_changed",_, socket) do
  IO.inspect("received!")
end

I also tried using touch_scroll instead of execute_script.

Any idea what could explain this behaviour?