Vertical scrollbar is broken on Linux in Tor Browser 8 with Gtk3

From the blog (https://blog.torproject.org/comment/276915#comment-276915):

Vertical scrollbar behavior is broken (Linux Mint, XFCE). This has been a known issue with GTK that is resolved by creating a settings.ini file in .config/gtk-3.0/ (see: https://support.mozilla.org/en-US/questions/1120904). This fix doesn't work in Tor Browser 8.0.

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