I've just upgraded from Zorin 16 to 17. And just like last time, I had non-working scrollbars in Thunderbird.
Note: I re-installed rather than upgraded because of using non-standard repositories. You need the Zorin repository version of Thunderbird for this to work.
Here's the thread from last time: Fix (remove) overlay scrollbars
And here's what I did this time...
Thunderbird settings changed:
widget.gtk.overlay-scrollbars.enabled: false
widget.non-native-theme.gtk.scrollbar.allow-buttons: true
widget.non-native-theme.scrollbar.size: 18
widget.non-native-theme.scrollbar.size.override: 18
~/.config/gtk-3.0/gtk.css:
Added this:
scrollbar {
-GtkScrollbar-has-backward-stepper: true;
-GtkScrollbar-has-forward-stepper: true;
}