code: def bookmarks_index_changed(self):
          
          index = self.bookmarks_list.currentIndex()
          if index >= 0:
              self.tool.reset()
              rectangle = self.bookmarks_list.itemData(index)
              self.tool.set_rectangle(rectangle)
              self.canvas.setExtent(rectangle)
              self.ok_button.setEnabled(True)
          else:
              self.ok_button.setDisabled(True)
docs: Update the UI when the bookmarks combobox has changed.