Here the GTK3 version of interlink (interlink-52.9.8194.linux-x86_64-gtk3_en_UK_rava_ln-interlink-bin.xzm), I zoomed into an email so that its content cannot be fully displayed, creating a highly visible scrollbar on the right and on the bottom:

And this is the same version of Interlink, but its GTK2 variant (interlink-52.9.8194.linux-x86_64-gtk2_en_UK_rava_ln-interlink-bin.xzm), same Email, same zoom level, but the scrollbars on the right and bottom are hardly visible:

Here is a zoomed in screenshot of Palemoon-32.4.1.linux-x86_64-gtk2 enhanced using Unsharp Mask:

By using that level of zoom and enhancing the screenshot with Unsharp Mask the scrollbar and the background are distinguishable. The scrollbar ends approx below the ")" of the text above.
But without that level of ridiculous zoom (that would be highly impractical) the scrollbar is hardly to be seen.

Any idea why that is? How can it be fixed?
I have the extra module libgtk+libgdk-x11-2.0.so.0.2400.33.xzm loaded, could that module be causing the issue?
(I wonder if that module is even necessary in Port 5.0.1, I needed it for at least one of my programs in Port 5.0)
But like I wrote above: the issue is also present in Port 5.0, it is not an 5.0.1 issue.
Update solved via this post:
Scrollbar hardly visible: GTK2 program in XFCE 4.16 and Port 5.0 or 5.0.1 and standard dark theme (Post by ncmprhnsbl #96345)
and reported what I added to my system in this post
Scrollbar hardly visible: GTK2 program in XFCE 4.16 and Port 5.0 or 5.0.1 and standard dark theme (Post by Rava #96346)