Hello Kiro!
The problem is given by js smoothscroll.
Note this, that point you to resolve: if you open/reload in chrome, and click on scroll bar, maintaining mousedown on scrollbar, scroll until bottom and maintain mousedown:
you see that the problem not appear, until you do not release the mouse.
Then, this appear in chrome console:
SmoothScroll.js?assets_version=171:1
Unable to preventDefault inside passive event listener invocation.
This not happen in firefox, then the js code should may be fixed to work fine on different browsers.
your smoothscroll on eles phpBB theme do not want leave you in peace, since last time you had some other problem with it if i'm not wrong!
Have you try to disable the smothscroll and check that the problem disappear?