New Window doesn't work
Using either my personal macbook or my mac mini at work, trying to create a new window through the File menu does nothing. Command-N will work, but it's odd that the menu option is buggy.
Works fine for me on MBA 2011 and MBP 2017 both running latest stable OSX.
Make sure you're running the latest stable version of macOS and the latest stable Vivaldi. If not, update. Updates are good.