Since Jenkins 2.2, the execute shell textarea overflows its container (see attached).
Horizontal UI layout in Job Configure stretches horizontally. Text no longer wrapped automatically in build step text boxes.
Actually, it did do that before, except the container grew with it, breaking the entire layout.
The behavior you're currently seeing is by design, as it's the least broken behavior possible.
(gus reiber Tom FENNELLY Could you confirm or dispute what I wrote here?)
Duplicates JENKINS-27367, which has been fixed in 2.26 and then backported to 2.19.4