Possible solution for Vivaldi Download issues
Using Windows 10, I've had occasional problems with Vivaldi downloading files. Perhaps 10% of the time the download would fully download but never complete and close the file. Just eternally continue despite having downloaded all the bytes it was supposed to download. I could immediately resolve the issue by downloading in another browser.
An attempt to replicate the issue the following day would normally fail, and Vivaldi would then download the file properly. So I could not provide a way to replicate the error.
About 8 weeks ago I turned off Windows defender's Smart Screen App and File protection. This Windows service checks unknown file downloads to help protect the user. So, it is likely to check new file updates, if it hasn't seen them yet.
Since that change, Vivaldi has not had any issue with downloads. So, it is possible that Vivaldi is not properly communicating with the Smart Screen app and file protection in Windows 10 Windows defender.