In a world of popup modals, popup windows seem to be not a priority to be fixed.
They open but are empty. If you launch it a second time, it works.
If you found a bug, please report the issue to the developers to fix it.
How to do a bugreport for Vivaldi
Report each bug seperately at https://vivaldi.com/bugreport/
Describe the issue as precise as you can.
Add some information about extensions you use.
Add information about the operating system and version you use.
If you have problems with video or audio give us information about your installed codecs and graphics card/GPU.
Add a exact link to the page where the issue comes up and can be tested.
After reporting the issue, you will receive a confirmation mail from the bugtracker.
In a reply to this mail you can add more information and files (like screenshots or testcases) as attachments and it will be added automatically to bug report.
If you have questions about the bug please ask in forum first and do not forget to post the VB-XXXXX bug number (you can find it in confirmation mail).
Thanks in advance for helping us to make Vivaldi better!