I was writing some code and noticed that col_idx does NOT exist as a property of ReadOnlyCell, but it does for normal Cell instances. I wrote a workaround:
getattr(cell, "col_idx", cell.column)
But, obviously, it would be preferable to just be able to retrieve col_idx.
Furthermore, cell.column in ReadOnlyCell is a number, whereas in normal Cell instances is a letter.
I may try to submit a patch later, but I wanted to have this logged in the meanwhile.
I didn't try this in the latest version of openpyxl, but looking at the source in master it seems to have not been addressed yet.