Refinements, "Show More" link stopped working

I recently discovered that the “Show More” link on the Search Refinement Panel web part 
had stopped working, seemingly out of the blue.  
With a bit of debugging I discovered that the functions defined in the onclick event:
onclick="SearchEnsureSOD();ToggleRefMoreLessFilters(this, true);"
were returning “Object Expected”.  
For some reason the functions were no longer defined.

I eventually figured out that the problem was that I had removed the Search Box web part 
from the page and replaced it with my own custom search box.  

Apparently the Search Box web part is required on the same page as the Refinement Panel web part,
as it injects JavaScript into the page that is used by the Refinement Panel.  

I fixed the problem by adding the Search Box web part back onto the page and hiding it.