I met the same problem on Xfce 4.14. Maybe it's a GTK+ notebook issue.
You can make all Geany's notebooks (Editor, Symbol list, Message window) scrollbar on the right edge by: `echo "#GeanyMainWindow notebook { border-right: 0; }" | tee -a ~/.config/gtk-3.0/gtk.css`
BTW, if you want reduce the size of the statusbar, do this: `echo "#GeanyMainWindow statusbar { margin: -10px 0; }" | tee -a ~/.config/gtk-3.0/gtk.css`