"Handlebars.compile is not a function" reported after upgrading from AEM 6.5.2 to AEM 6.5.6

I have recently upgraded from AEM 6.5 SP 2 to AEM 6.5 SP 6.

Since upgrading, the Select Font button on Rich Text Content Fragment editor has stopped working, and I am seeing the error "Handlebars.compile is not a function" in console output.

Solution:
It looks like AEM 6.5 SP 6 may have updated the handlebars version within /libs/clientlibs/social/thirdparty/handlebars. What you need to do is find out which version AEM 6.5 SP 2 uses, and Override the client library for the correct version. The clientlib.category should be cq.handlebars.