Cluster selection should be limited to the currently active voice. By "active voice" I mean the voice indicated by the "V1" / "V2" callouts / buttons in the floating measure toolbar when a measure is selected. When neither of them is active (they grow larger in size when selected/activated) then no cluster should be selectable. Hovering clusters should not make their hotspot visible, and clicking on them should select the parent measure instead. The same should happen when hovering the clusters of the other voice. Selecting clusters that live in the currently active voice should work as expected.