I discovered and pressed the "Hide Code" button on the right margin of the editor while working on a .mlx file.
It works unidirectionally: I can't unhide the code.
This state appears to be persistent: Restarting MATLAB and OS has no effect.
Restarting MATLAB using an historical editor-state, also has no effect, suggesting the view state is bound to the file. (It also suggests I've accidentally saved the .mlx file.)
Further, the reopened file flags the following warning:
I also tried to open/save script using an earlier release. The code fields reappear in R2018b, but saving the .mlx file doesn't change the hidden state when re-opening in R2019a.
Aside: The «hide code» button appears to act like a radio button grouped with the two view-options for placement of output. (This seems an odd choice.) So activating one of the other two, changes the target button-state, but not the view-state.
R2019a + macOS 10.14.4