Selenium tests fail with azure pipeline sometimes and pass locally - selenium

Question

Test cases fail while not finding a link or any validation messages after a button click.
I used explicit waits for page to load:
var waitForDocumentReady = new WebDriverWait(WebDriver, TimeSpan.FromSeconds(10));
waitForDocumentReady.Until((wdriver) => (WebDriver as IJavaScriptExecutor).ExecuteScript("return document.readyState").Equals("complete"));
For wait specific div of validation messages:
WebDriverWait wait = new WebDriverWait(WebDriver, TimeSpan.FromSeconds(10));
wait.Until(SeleniumExtras.WaitHelpers.ExpectedConditions.ElementExists(By.ClassName("validationErrors")));
But test cases pass sometimes with pipeline and fails sometimes.

Answer 1

I would recommend following:
When ever your tests fail take a snapshot at that moment.
sometimes when you run your tests locally the network communication
is a bit faster than running on server. Which means a spinner or a
loading control may not appear locally because results were retrieved
fast enough but the loader will appear when there is delay from
server in sending the response to client.
If you are trying to run your tests in FF headless on remote grid
setup then it is not yet stable in my experience.
Identify the pattern in failed scenarios which frequently fail and
try and run them multiple times in a loop and identify the pattern of
failure. In one of my case there was an actual bug in the performance
of the application which used to get reproduced whenever the tests
ran immediately in bulk

Answer 2

I was having this issue, except my test would always fail in Azure Pipelines but pass locally. Not running in "headless" mode fixed my problem. To do this, make sure "Test mix contains UI tests" is checked in your "Visual Studio Test" task.

Answer 3

There are a couple of things you need to take care as follows:
Test cases fail while not finding a link or any validation messages after a button click: A lot depends on the attributes of the button. In case the button contains either of the following attributes/events the DOM Tree would be getting updated and subsequently trying to find the desired element may fail:
onclick() event:
<button onclick="myFunction()">Click me</button>
jQuery click() Event:
$("p").click(function(){
alert("The paragraph was clicked.");
});
So after a button click, to find the desired element you need to induce you have to induce WebDriverWait for the ElementExists().
You can find a relevant discussion in ExpectedConditions.ElementIsVisible returns TimeoutException even when element is present
ExecuteScript("return document.readyState").Equals("complete"): This line of code is not that effective to ensure if the desired element(s) are present/visible/interactable with in the HTML DOM.
You can find a relevant discussion in Do we have any generic function to check if page has completely loaded in Selenium
However to wait for specific div with ElementExists() should have worked. The relevant HTML would have helped us to analyze what went wrong. However, if the validationErrors related element is based on HTML5 Constraint validation then you have to use the GetAttribute() method.
You can find a relevant discussion in How to handle html5 constraint validation pop-up using Selenium?