Can't drag cards in Vivaldi

[Svashta]

I am using fizzy.do, which probably uses this repo, too - but I did not cloned it and tried locally...

For some reason, most of the time I can't drag cards on Vivaldi. I have tried other browsers , and it works fine there (Chrome for example). I thought it is maybe a combination of Extensions, but even after I've disabled all of them, still can't drag cards.
I have also disabled Vivaldi's content blocker.

There are no errors in the console - it just doesn't register the click-drag I guess.

Just wanted to let you know, and if there is any remote possibility to make it work, I would really appreciate it.
It drives me crazy sometime and then I just open Chrome and do it there.