(Note that if the frame is shrunk so that not all marked file names fit
in the *Marked Files* window, then without an oversized header line as
is my-mm, dired-pop-to-buffer fails with the error "Attempt to delete
minibuffer or sole ordinary window"; whereas with an oversized header
line, this error is raised only if not even one file name can be shown.
But I see this difference in behavior whether or not the above patch is
applied.)

Steve Berman