WFM. But only when the sidebar scrollbar is visible under the insane hiding behaviour of GTK (sidebar has a normal GTK scrollbar). The editor sidebars are controlled by Scintilla and follow Scintilla scrollbar behaviour which is only slightly eccentric (ie not as insane as GTK). I guess its the least common behaviour Scintilla can use in common between Qt, GTK, Windows, and Macos.
Perhaps GTK CSS has the ability to control the hiding behaviour, but I don't know it. Perhaps ask on a GTK CSS forum.