When I try to solve a more complex merge I often want to see a diff between an ancestor and a source or between a source and a destination. I know that the merge tool theoretically shows all those diffs at once, but it is not so legible. It would be nice to hide one of the file temporarily and see clean diff between the remaining two only. Or at least open those simple diffs from a context menu in a merge tool, as I can from an "items to merge" list.