Steps to reproduce (in Firefox):

1. Go to https://de.wikipedia.org/wiki/Element_(Auszeichnungssprache)?veaction=edit.
2. Edit the second syntaxhighlight-element in the section "HTML (Version 1 bis 4)"
3. Note there is an "i" left of the first line of code (probably it wants to inform me about the unclosed tag).
4. Hover over this "i". Note that a scrollbar on the dialog is shown (perhaps some more information about the fact that the tag is unclosed is shown below).
5. Try to scroll. Once you start scrolling, you stop hovering, and the scrollbar vanishes.

In Vivaldi, in step 4 no scrollbar appears, nor does happen anything else. If hovering over the "i" is meant to show additional information, this is equally bad, and makes the "i" useless in complex situations.