We've received feedback that the combined sorting & grouping functionality on Master/Detail view (using the "Sort" menu) is not granular enough, and that it would be better to have separate Sort & Group settings like the List View does. This would also allow you to apply a sort without any grouping, which could be useful in some cases. What do you think?
On a side note - a workaround for sorting and grouping on different fields is to use Advanced Sorting on these types of pages. The first sort field will drive the grouping, and then the items within each group will be sorted by the second sort field you select.
