In that forum it says that you must edit the parameters of the command line with which Vivaldi is launched by your graphic desktop to get scrolling, if that is what you want.

I am pretty sure that the middle mouse button is not broken in Vivaldi on Linux.

It probably just has a different default behavior than what you expect, e.g. the middle button might have been allocated by default to mouse gestures, not to clipboard or scrolling or whatever you expect.

I am using Vivaldi, but I do not remember which was its default behavior, because I no longer use mice, but a stylus on a small graphic tablet, and I reconfigured Vivaldi for the buttons of my Wacom stylus.

In any case, if I had been using a mouse, I would have wanted to allocate the middle button to mouse gestures, because those are much more useful than the uses of the middle button for the X clipboard or for scrolling.