Geany opened, maximises and sets pane to 260????? Thats not either of the previous positions.
It's not. Because "3. User moves sidebar to position 500." So the new sidebar size is 140. If instead of closing, I had gone on...
4. Geany unmaximized. GTK magic: Non-max state restored, 400x300. Sidebar position **260**. (This is the adjusted size that was saved in the previous scenario.)