This code shows how to trigger a Modal onScroll event to perform and assert against an ajax request
// Make the first request
$browser->visit('/page')
->clickLink('search') // Assuming this will open the Modal
->waitFor('.selector')
->assertSee('Text 1');
// Assert the next comming text is not there
$browser->assertDontSee('Text 2');
// Trigger the Modal scroll event to perform the second ajax request
// Note: The value ( 10 ) depends on how you handle the onScroll event
$browser->driver->executeScript('var el = $(".modal-content"); el.scrollTop( el.prop("scrollHeight") + 10 );');
// Wait until the Preloader disappear and make the assertion
$browser->waitUntilMissing('.preloader-wrapper')
->assertSee('Text 2');