"select all files" was just one key: * (actually, it was invert selection -- so assuming no files were selected beforehand). Pressing + and Enter would select one file, then try to edit/run the next one?

I believe pressing + should open "Select files" dialog prefilled to select all files (and dirs), pressing Enter confirms it.

My Midnight Commander 4.8.33, however, remember previously entered mask, and if no mask was entered - then it defaults to selecting nothing :(