OneNote's box-outlines and text caret randomly become unresponsive on Sierra and High Sierra
I found a consistent workaround for myself:

Three-finger-swipe up (or any equivalent, to see your Spaces), select any other application, and OneNote should be normal again when you come back.

This issue may (or may not) be occurring due to using multiple desktops (Spaces), and possibly due to having multiple OneNote windows open (on different desktops).