The visibility of the scrollable Arranger index introduced by version 3.25-10 may be switched off or on via the menu item "Diagram => Show Arranger index?" (from version 3.25-11 on):

If enabled then the index tree of all currently arranged diagrams by groups will appear aside the work area unless the Arranger doesn't contain any diagrams or the Structorizer instance isn't associated (i.e. doesn't listen) to the Arranger:

See the Arranger index section on the Arranger manual page for details how to work with the Arranger index.