getTileHoldersByGroupEvent
Collects every TileHolder in this subtree (this one, then recursively through tiles) whose handlesGroupEvent returns true for event — the traversal behind TileRenderingScope.dispatchGroupEvent, which is how RadioButton's mutual exclusion reaches every radio button sharing a groupId regardless of where each one sits in the tree.
Return
every matching TileHolder in this subtree, in tree order.
Parameters
event
the group event to match tiles against.