1
this might be a Mac only regression. in v5 for mac i can certainly confirm that this was still possible. i am not sure about v6, but v7, v8 and v9 does not allow this anymore, so please enable viewport manipulation with the display colour dialogue in the layers again.
2
also while we are at it, viewport manipulation while having the layer material dialogue open should also be possible, that is quite essential to zoom into parts that need material adjustment, otherwise its always material on fiddle around, close the dialouge, zoom around, open dialogue again etc etc etc.