GtkFileChooserButton entries should have tooltips that include the full path
When selecting a folder comparison on the opening page, the dropdowns have an entry that appears to let you select an item from the history.
The full path isn't shown, though, and (unlike the Open Recent submenu) there's no tooltip when hovering.
This is a bit unhelpful when comparing related folder structures. It's very common - at least for me - to compare folders that have the same name as one another.