I had a Problem, but in this case I found some solutions here and on other websites I want to share with you.
Problem: window.opener functions of child pages were not working anymore since some security updates of the modern browsers about 2020 / 2021.
If you open a new link from your site via target="_blank" then in most cases the window.opener functions will not work anymore.
Solutions: see Answer below. I hope I could help other people with this solution.
I have searched many hours to solve these problems. I have even tried to allow the old behaviour via .htaccess but without success. If anybody has a working solution via .htacces please add a post or an answer.
For simple href links you just have to add rel="opener" to get it working again.
<a href="https://myPage.com/childpage.php" target="_blank" rel="opener">Link to child page</a>
For some JavaScripts (one sample here) you can add setAttributs to your scripts
(...)
var form = document.createElement("form");
form.setAttribute("method", "post");
form.setAttribute("action", "myChildPage.php");
// form.setAttribute("target","_blank"); // old
form.setAttribute("target", "formresult"); // new: setting form target to a window named 'formresult'
form.setAttribute("rel","opener"); // need to add because of security changes of newer browsers
(...)
document.body.appendChild(form);
form.submit();
For some Buttons with input type="submit" this is not as easy as the solutions before... The rel="opener" does not work within the button if you use formaction, formtarget (...)
Not working (rel="opener" in INPUT tag):
<form name="myForm" id="myForm" method="post">
(...)
<input type="submit" value="My Submit Button" formaction="childpage.php" formmethod="post" formtarget="_blank" rel="opener">
</form>
Working (rel="opener" in FORM tag)
<form name="myForm" id="myForm" method="post" rel="opener">
(...)
<input type="submit" value="My Submit Button" formaction="childpage.php" formmethod="post" formtarget="_blank" rel="opener">
</form>