Menu rows survive a file read mid-write
Pull request 16268, merged into quattro, keeps the menu's rows when a read is not a whole menu. A half-written file had been replacing every row, and the menu could stay empty until the shell restarted.
An empty read of the shipped menu counts as a mid-write too, because that file is never legitimately empty. An empty or comments-only user extension still clears its rows. Opening the menu when it has no shipped rows reads the file again.
David Heinemeier Hansson wrote that with the shipped menu file left half-written on disk, the running menu still opened with every row.