Hello folks,
You can blame Nick for that one Iain. For datasets with largish numbers of sub-bricks the program turns to a different type of menu selection. The trigger for the switch is controlled by the environment variable: SUMA_ArrowFieldSelectorTrigger that would reside in your ~/.sumarc (update whatever you have with suma -update_env ) then change that env from its default of 200.
The arrow field selector is more handy when you have a very large number of sub-bricks. In Nick's case he also wanted SUMA to automatically switch to certain sub-bricks whenever he selected a different node. The intent there was to display connectivity data.
cheers,
Ziad