Comment 7 for bug 1691678

Is this an upstream issue (does Fedora show that behavior as well?) or the outcome of a Debian patch? There is at least one which is scrollbar related:

+ Add scrollbar-background-theming.patch:
      - Draw background under the scrollbar that matches the actual terminal
        background color. This allows proper theming.

https://lists.canonical.com/archives/cosmic-changes/2018-September/009594.html