Hi,
After upgrading to moodle 3.4.2+, we have an undefined window popup in every page visit.
server php version: 7.2.5 On centOS 7
Which version of Moodle did you upgrade from?
And do you have any third party plugins installed? If so, have you tried disabling them?
From memory this issue occurs when Moodle (or a plugin) tries to do something in the client-side JavaScript which results in an unhandled error.
You could check in your browser to see if there are any failed AJAX requests or any JavaScript errors reported in the console that coincide with this issue.
This can be hard to troubleshoot depending on your experience. Let us know how you get on or if you need further help with this.
Just to clarify... open your Javascript Console (in Chrome, for example, View > Developer > JavaScript Console).
Reproduce the error. Do you see any error messages in the console? They will be in red. You'll probably get some other messages anyway. It's also worth trying with Debugging turned on in Moodle. You may have to enable debugging by adding the lines from config-dist.php into config.php.