Reset default track zoom in Logic Pro X
The default track zoom is controlled by a little slider in the upper right corner just above the tracks area:



Clearly, I must have accidentally touched the mouse button while the pointer was above that slider. Once that slider is set to a sensible value, everything behaves normally, and shift-double-click on a track header border resizes all tracks to that sensible height. 

However, they will never behave quite normally again. A sad and inexcusable mess on the Logic team's part. 

The automation issue must be unrelated.