One of the settings that are now enabled when updating your Vivaldi installation to 7.1 and up is, that the browser won’t stop running completely when closed, which is a problem e.g. when you have full-screen applications like gaming and the M3 mail notfications of new emails coming in keep popping up, or the bug that POP3 emails no longer are populated into your inbox anymore, appears.
Rather than always manually kill the process using Windows task manager, you can disable this behavior!
I found the solution thanks to this thread on reddit, however it’s outdated.
The current way of finding this setting and disabling is, is, going to the address
chrome://settings –> settings, there you can find the option “Continue running background apps when Vivaldi is closed”, disable it.