Vivaldi vs Oscar Editor
there's a compatibility problem with Vivaldi and Oscar Editor.
Oscar Editor is a driver/tool for A4Tech mice. It allows the user to configure macros and other b/s advanced features of decent but not too fancy mice. It also has a feature allowing the user to use horizontal scrolling with only 1-wheel mice. Scrolling switches to horizontal when you move to the side of an active window. In Vivaldi, no matter how its window is configured (maximalized or small window somewhere on the screen), the advanced scrolling function is always set to horizontal scrolling breaking the function altogether. Switching to any other non-Vivaldi window makes this issue disappear.
I'm using the latest official, automatically updated release of Vivaldi on a 64bit Windows7.
This issue has been here for at least several versions of Vivaldi. It's hard to keep thrack of it with updates so frequent.
Have you tried checking "Use Native Window" in vivaldi://settings/appearance/? I'm not sure if this will resolve your issue, but I think it's worth a shot... 😜