Sierra USB mouse wheel random zooming
Something that has started happening since installing Sierra: occasionally when I'm scrolling with the mouse wheel (it's a Microsoft mouse if that makes any difference), Vivaldi randomly zooms in/out. It continues to scroll the page has normal, but with the added "effect" of zooming. It is also not something that happens all the time, but hammering the mouse wheel definitely causes issues.
It looks like your issue is the same as the one already discussed here. In that thread, people are reporting that installing the macOS update 10.12.1, which was released yesterday, fixes the issue.