> I double-checked, and the code I changed, dired-mark-unmarked-files, is
> only called by dired-omit. However, it can also be called interactively,
> so I have certainly changed the interactive behavior.
It can also be called from 3rd-party code.
​Quite right, and the suggestion I make retains the same behavior (as the new argument I suggest is optional and its default value keeps the current behavior).