Essentially, a user's answer in Excel (now deleted) uses the drag-down formula feature of Excel, as described in the linked related meta question. I thought that from the highest voted answer to that question, we had, by community consensus, determined that mouse interaction is unnecessary. I personally take it to mean that mouse interaction of that type is not a valid form of "input" by default.
There is no related "input" type associated with mouse input listed in the default I/O methods in the relevant meta question. I don't think the mouse should be used to in any way affect the program's structure.
As we said, this allows an arbitrary amount of rows or columns in this case, so what if I create a language where the numbers of rows or columns I drag determines the output entirely?
To be clear, Excel only happens to be the catalyst for this question, but there are plenty of other situations where mouse input is possible -- Minecraft, Terraria, Logisim, LabVIEW, HTML+CSS, Scratch ...
So, to remove any doubts, what mouse interaction is allowed to be used for input/output?