Swipe operation wasn't possible
When I updated it in Vivaldi 1.5.658.56, swipe operation wasn't possible with a magic mouse. The gesture operation doesn't function even if I turn on a gesture by setting.
The Vivaldi 1.6 release candidate contains fixes for swipe gestures on the Mac. I confirmed that they work with both the trackpad and Magic Mouse, at least on OS X 10.11.6.
I don't have Vivaldi 1.5 installed at the moment so I can't confirm whether or not it's broken for me on this release. However, a fix for you (hopefully!) should be available very shortly.
Correction: Swipe navigation works intermittently in Vivaldi 1.6 RC!
It was working fine then it suddenly stopped working! If I open a page in a new window and click some links to generate history, it starts working again.
I've reported the issue and hopefully they can get this working reliably again soon.