Some other useful pieces of information:

This user is describing something similar but it's for a program that I don't use. My guess it that his program uses the same underlying engine and thus is subject to the same bug.
https://github.com/jupyter/notebook/issues/4138

This is not related to the new OSX Mojave upgrade. I have been having the problem for a couple of months, and I only upgraded to Mojave last night, and it didn't fix or change the problem.